^0  8 
©CCGL 


UNIVERSITY  OF  CALIFORNIA  PUBLICATIONS 

IN 

AGRICULTURAL   SCIENCES 

Vol.  3,  No.  11,  pp.  283-368,  plates  25-42,  9  text  figures  April  4,  1919 


AN   INVESTIGATION  OF  THE  ABNORMAL 

SHEDDING  OF  YOUNG   FRUITS 

OF  THE  WASHINGTON 

NAVEL  ORANGES 


BY 

J.  ELIOT  COIT  AND  ROBERT  W.  HODGSON 


UNIVERSITY  OF  CALIFORNIA  PRESS 
BERKELEY 


TJNIVEESITT  Or  CAlirOENIA  PUBLICATIONS 

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addressed  to  THE  EXCHANGE  DEPARTMENT,  UNIVERSITY  LIBRARY,  BERKELEY, 
CALIFORNIA,  U.S.A. 

AGRICULTURAL   SCIENCES. — Charles   B.    Lipman,    Ernest   B.    Babcock,    and   John   W. 
Gilmore,  Editors.    Price  per  volume,  $5.    Volume  1  (587  pages)  completed.    Volumes 
2,  3  and  i  in  progress. 
Vol.  8.    1.  Studies  tn  Juglsns  I.     Study  of  »  New  Form  of  Juglans  Californiea 
Watsoii,  by  Ernest  B.  Babcock.     Pp.  1-46,  plates  1-12.    December, 

1913 „ _ .. .60 

2.  Stndleg  in  Juglans  n.    Further   Observations   on   a  New  Variety   of 

Juglapi  Cahfomiea  Wataon   and  on  Certain   Supposed  Walnut-Oak 
Hybrids,  by  Ernest  B.  Babcock.     Pp.  47-70,   plates  13-19.     October, 

1914  ..„ „ - _ - - - 86 

S.  Studies  iQ  Juglans,  HI:  (1)  Further  Evidence  that  the  Oak-like  Walnut 

Originates  by  Mutation,  by  Ernest  B.  Babcock.    Pp.  71-80,  pis.  20-21. 

September,  1916 _ _ _ _ _ — —      .10 

VoL  3.    1.  New  Grasses  for  California,  I,  Fhalaris  stenoptera  Hack.,  by  P.  B.  Ken- 
nedy.   Pp.  1-24,  plates  1-8.    July,  1917 30 

2.  Optimum  Moisture  Conditions  for  Young  Lemon  Trees  op  a  Loam  Soil, 

by  L.  W.  Fowler  and  C.  B.  Lipman.     Pp.  25-36,  plates  9-11.     Sep- 
tember, 1917  .15 

3.  Some  Abnormal  Water  Relations  in  Citrus  Trees  of  the  Arid  South- 

west and  their  Possible  Significance,  by  Robert  W.  Hodgson.     Pp. 
37-54,  plate  12.    September,  1917  -       .20 

4.  A  New  Dendrometer,  by  Donald  Bruce.    Pp.  55-61.    November,  1917 10 

5.  Toxic  and  Antagonistic  Effects  of  Salts  on  Wine  Yeast  (SacchaTomyces 

eUipsoideus) ,  by  S.  K.  IKUtra.    Pp.  63-102.    November,  1917  .45 

6.  Changes  In  the  Chemical  Composition  of  Grapes  during  Ripening,  by 

F.  T.  Bioletti,  W.  V.  Cruess,  and  H,  Davi.    Pp.  103-130.    March,  1918      .25 

7.  A  New  Method  of  Extracting  the   Soil  Solution    (a  Preliminary   Com- 

munication), by  Chas.  B.  Lipman.     Pp.  131-134.    March,  1918 .05 

8.  The  Chemical  Composition  of  the  Plant  as  Further  Proof  of  the  Close 

Relation  between  Antagonism  and  Cell  Permeability,  by  Dean  David 
Waynick.    Pp.  135-242,  plates  13-24.     June,  1918 1.25 

9.  Variability  in  Soils  and  Its  Significance  to  Past  and  Future  Soil  In- 

vestigations.   I.  A  Statistical  Study  of  Nitrification  tn  Soil,  by  Dean 
David  Waynick.    Pp.  243-270,  2  text  figures.    June,  1918 SO 

10.  Does  CaCOa  or  CaS04  Treatment  Affect  the  Solubility  of  the  Soil's 

Constituents?,  by  C.  B.  Lipman  and  W.  F.  Gerlcke.     Pp.  271-282. 
June,    1918    10 

11.  An  Investigation  of  the  Abnormal  Shedding  of  Young  Fruits  of  the 

Washington  Navel  Orange,  by  J.  Eliot  Coit  and  Robert  W.  Hodgson. 

Pp.  283-368,  plates  25-42,  9  text  figures.     April,  1919  1.00 

Vol.  4.    1.  The  Fermentation  Organisms  of  California  Grapes,  by  W.  V.  Cruess. 

Pp.  1-66,  plates  1-2,  15  text  figiires.    December,  1918 75 

2.  Tests  of  Chemical  Means  for  the  Control  of  Weeds.    Report  of  Progress, 

by  George  P.  Gray.    Pp.  67-97,  11  text  figures (In  press) 

3.  On  the   Existence   of   a   Growth-Inhibiting   Substance  in  the  Chinese 

Lemon,  by  H.   S.  Reed  and  F.   F.  Halma.     Pp.  99-112,  plates  3-6. 

February,  1919  25 

AGRICULTURE.— The  Publications  of  the  Agricultural  Experiment  Station  consist  of  Bul- 
letins and  Biennial  Reports,  edited  by  Professor  Thomas  Forsyth  Hunt,  Director  of 
the  Station.  These  are  sent  gratis  to  citizens  of  the  State  of  California.  For 
detailed  information  regarding  them  address  The  Agricultural  Experiment  Station, 
Berkeley,  California. 
BOTANY. — W.  A.  Setchell,  Editor.  Volumes  I IV,  $3.50  per  volume;  volume  V  and  follow- 
ing, $5.00  per  volume.  Volumes  I  (pp.  418),  n  (pp.  360),  in  (pp.  400),  and  IV  (pp. 
397)  completed.  Volumes  V,  VI,  and  VII  In  progress. 
Vol.  3.    1.  Compositae  of  Southern  California,  by  Harvey  Monroe  HaU.     Pp.  1- 

302;  plates  1-3,  with  a  map.    December,  1907 3.00 

2.  The  Origin,  Structure,  and  Fimction  of  the  Polar  Caps  In   Hmilarina 
timiih:xiranli«  Nutt.,  by  H.  D.  Densmore.     Pp.  303-330;   plates  4-8. 

December,  1908  : 35 

3,  4.  (In  one  cover.)  The  Value  of  Sodium  to  Plants  by  Reason  of  Its 
Protective  Action.  On  the  Effects  of  Certain  Poisonous  Gases  on 
Plants..  By..W..J,  .V,  Ost«rhcut.'-Pp.-83i-340.-.  .J^ijie, .19D8: 10 


UNIVERSITY  OF  CALIFORNIA   PUBLICATIONS  /5^^C/6>^ 

IN 

AGRICULTURAL    SCIENCES  CJ^\p.\ 

Vol.  3,  No.  11,  pp.  283-368,  plates  25-42,  9  text  figures  April  4,  1919 


AN   INVESTIGATION   OF   THE   ABNORMAL 

SHEDDING  OF  YOUNG  FRUITS  OF  THE 

WASHINGTON  NAVEL  ORANGE* 

BY 

J.  ELIOT  COTT  AN-D  ROBERT  W.  HODGSON 


Introduction 

Tlip  geini.s  Citrus  is  undonhtpdly  of  tropical  origin.  Alphonsp  de 
Candollo.  aftor  much  investifration  of  liistorical  and  pliilological  data, 
concludes  that  the  fcrnl  range  of  the  sweet  orange  is  South  China, 
Cochin  China,  Java,  and  Sumatra,  with  a  possible  extension  into  India, 
which  regions  are  cla.ssed  ecologically  as  tropical  rain  fore.st.  ^lorpho- 
logical  evidence  of  the  tropical  origin  of  the  orange  is  abundant,  its 
tropical  mesopliytic  nature  licing  indicated  by  glossy,  broad.  Ihit  leaves 
of  rather  loose  and  open  cell  .structure,  long  life  of  leaves,  absence  of 
stoniatal  devices  for  regulating  transpiration,  lack  of  root  hairs,  and 
lack  of  a  regular  and  non-interruptable  period  of  dormancy.  Living- 
ston' ha.s  recently  pointed  out  that  tlie  most  eificient  climate  for  plant 
growtli  in  the  United  States  is  ])eninsular  or  trojiical  Florida.  The 
significance  of  this  is  apparent  when  we  remember  that  tropical 
Florida  is  the  only  place  in  the  United  States  where  the  orange  has 
run  wild  and  been  able  so  to  maintain  itself.  In  all  countries  where 
the  sweet  orange  has  nm  wild  after  having  been  introduced  into  the 
Xew  World,  such  as  Brazil.  Paraguay,  northern  Argentina,  and  to 
some  extent  in  i^'lorida.  the  climate  is  distinctly  tropical. 

Ilorticulturi.sts  have  called  attention  to  the  fact  that  an  environ- 
mental complex  which  is  most  efficient  as  regards  plant  growtli  does 
not  nece.ssarily  conduce  to  the  production  of  fruit  of  high  connncrcial 
\aliie.    (In  llie  otlicr  hand,  some  rliiiialic-  factors,  sudi  as  lighl  and  heat, 


•Manuscript  .submitted  January  17,  IIMS. 
1  Physiol.  Rea.,  vol.  1,  April,  1916. 


210063 


284  Universiti/  of  California  Publications  in  Agricultural  Sciences      [Vol.3 

wliifli  in  cxcessivo  amounts  tend  to  retard  vegetative  growth,  intensify 
certain  characteristics  of  the  fruit  which  greatly  enhance  its  market 
value.  Thu.s  w(>  find  that  the  Bahia  or  Wa.shington  Navel  variety  of 
Citrus  sinensis  has  comparatively  little  commercial  value  at  Bahia. 
Brazil,  where  it  originated,  or  in  any  other  tropical  country  where  it 
has  been  tested.  In  a  semitropical  desert  environment,  however,  this 
variety  of  orange  i.s  high  in  sugar  content,  has  skin  characteristics 
which  les.sen  decay  in  transit,  and  is  possessed  of  a  deep  reddish  orange 
color  which  increases  its  salability.  For  these  reasons  the  cultivation 
of  oranges  under  arid  and  semiarid  conditions  has  developed  into  an 
industry  of  large  importance,  in  which  many  millions  of  dollars  are 
invested  and  upon  which  many  thousands  of  people  are  dependent  for 
a  livelihood. 

When  we  consider  the  morphological  characteristics  of  the  more  or 
less  xerophytic  vegetation  indigenous  to  the  region  now  occupied  by 
orange  orchards  in  California  and  note  the  sti-iking  dissimilarity 
between  the  forms  of  native  plants  and  citrus  trees,  we  may  reasonably 
suspect  that  our  orange  trees  may  find  it  more  or  less  difficult  to  adjust 
themselves  to  the  new  and  strange  environment.  Perhaps  the  under- 
ground environment  provided  by  soils  which,  on  account  of  low  rainfall 
and  consequent  lack  of  leaching,  still  retain  a  large  proportion  of  the 
soluble  salts  resulting  from  the  decomposition  of  soil  minerals,  would 
be  equally  a.s  disordered  as  the  above-ground  enviromnent  were  it  not 
for  the  fact  that  water  artificially  applied  by  irrigation  lessens  the 
asperity  of  the  conditions  met  by  the  roots.  Not  only  is  the  total 
environmental  complex  to  which  our  orange  trees  are  exposed  incon- 
sistent with  their  natural  requirements,  but  the  trees  of  the  "Washing- 
ton Navel  variety  are  themselves  decidedly  abnormal.  It  is  the 
universal  practice  to  place  scions  of  the  desired  variety  ui^on  rootstocks 
of  other  species  of  Citrus  so  that  the  reciprocal  influences  between 
stock  and  .scion  come  into  full  play.  Moreover,  the  variety  in  question 
bears  some  indications  of  hybrid  origin.  The  blossoms  are  entirely 
devoid  of  viable  pollen,  functional  ovules  are  few.  the  fruits  are 
partially  double,  peculiar  in  structure  and  seedless,  and  the  vegetative 
parts  exhibit  an  erratic  polymorphism  which  has  so  far  proved 
decidedly  puzzling. 

It  is  a  matter  of  common  observation  that  in  the  interior  de.sert-like 
valleys  of  the  arid  southwest  the  Navel  orange  is  scmiewhat  dwarfed 
in  stature,  tiie  leaves  tend  to  persist  to  an  unusual  age,  the  volume  of 
bloom  is  abnormally  large,  shedding  of  the  flowers  and  yomig  fruits  is 


1919]       Coit-HodgsoH :  Abnormal  Shedding  of  Washington  Navel  Orange  285 

excessive,  ami  various  pliysiological  derangements  of  nutrititm  are  of 
frequent  occurrence. 

In  many  interior  localities  where  there  are  l)ut  few  pests  to  hinder 
the  growth  of  the  tree  and  where  the  climatic  conditions  favor  the 
production  of  cai-Iy  maturing  fruit  of  good  color  and  hiirh  sugar  con- 
tent, the  excessive  shedding  of  young  fruits,  or  "Juiu'  drop."  as  it  is 
called,  is  particularly  exasperating  to  growers,  who  would  undoubtedly 
make  much  greater  profits  if  some  way  could  be  devised  to  prevent 
that  part  of  the  drop  which  is  in  excess  of  the  normal  and  necessary 
amount.  An  investigation  of  this  problem  wa.s  undertaken  by  the 
writers  in  response  to  a  resolution  pas.sed  by  the  California  State  Fruit 
Growers'  Convention  calling  the  attention  of  the  university  authorities 
to  the  urgent  need  of  an  investigation  of  this  subject.  The  results 
secured  from  observations  and  experiments  during  the  summers  of 
191  ()  and  1917  are  brought  together  in  his  paper. 

Jlost  of  the  field  experiments  from  which  our  data  have  been 
obtained  were  carried  on  at  two  stations  in  Kern  County;  one  at 
Edison  in  the  orchards  of  the  Edison  Land  and  Water  Company,  about 
eight  miles  southeast  of  Bakersfield,  and  the  other  about  two  miles 
and  a  half  di.stant  at  East  Bakersfield  in  the  orchard  of  Dr.  C.  W. 
Kellogg.  Both  .stations,  on  account  of  being  situated  to  leeward  of  a 
considerable  stretch  of  desert  typical  of  the  southern  San  Joaquin 
Valley,  experience  the  extreme  climatic  conditions  referred  to  above. 
The  Navel  orange  matures  early  and  is  of  exeellent  (|uality.  and  wei-e 
it  not  for  the  light  crops  borne  this  district  would  he  considered 
excellent  for  the  production  of  Navel  oranges.  Under  these  climatic 
conditions,  unmodified,  the  drop  occurs  every  year  and  is  not  de- 
pendent on  the  occurrence  of  dry  hot  winds,  a.s  is  the  case  in  southern 
California. 

At  Edison  the  Navel  orange  trees  appear  healthy  and  \igorous,  the 
leaves  and  branches  being  quite  free  from  fungous  parasites  and  scale 
in.sects.  Except  for  an  occasional  slight  showing  of  mottled-leaf  disease 
the  trees  may  be  considered  very  thrifty  and  of  good  size  for  their  age. 
which  i.s  eigiit  years.  A  general  view  in  this  orchard  is  shown  in  plate  2o. 

The  soil  conditions  are  good.  The  type  is  Delano  sandy  loam  of 
good  deptii.  No  general  layer  of  hardi)an  exists.  Although  certain 
bodies  of  hard  conglomerate  occur  occasionally  these  are  not  in  layer 
formation  and  do  not  interfere  with  the  drainage.  The  soil  is  rich  in 
most  plant  foods,  though  low  in  iiiti'iiirii,  wliicli.  according  to  an 
analysis  kindly  made  by  Dr.  ('.  B.  Lipiiiaii.  runs  from  .0'2'i  per  cent  in 


286  University  of  California  Publications  in  Agricultural  Sciences      [Vol.3 

the  first  six  inches  to  .012  per  cent  at  a  depth  of  three  feet.  He  also 
reports  the  nitrifying  power  of  the  soil  as  fairh'  good  and  the 
ammonifying  power  as  high.  The  organic  matter  content  is  quite  low, 
much  lower,  in  fact,  than  one  would  suppose  from  the  healthy  appear- 
ance of  the  trees. 

Irrigation  water  is  pumped  from  wells  situated  on  the  tract  and  the 
irrigation  practice  follows  closely  that  of  southern  California.  Water 
is  applied  in  four  shallow  furrows  to  each  middle  about  once  a  month. 
This  is  followed  in  a  few  days  by  shallow  cultivation  in  both  direc- 
tions. The  amount  of  water  applied  is  sufficient  to  wet  the  soil  five 
feet  deep  and  throughout  the  whole  area  except  for  a  small  space 
between  the  trees  in  each  tree  row.  In  June  the  temperature  of  the 
water  as  used  is  about  75°  F.  Hilgard  advanced  the  idea  that  June 
drop  might  be  caused  l)y  low  temperature  of  the  irrigation  water. 
While  it  is  entirely  po.ssible  that  cold  water  may  influence  drop,  we 
have  found  the  drop  to  occur  regularly  where  the  water  was  not  cold. 


TABLE 

1 

Moisture  Determinations  in 

Edison  Soil 

Furr 

3WS  ruu  nortl 

and 

south 

Location  of  sample 

with  reference 

to  tree 

Depth 

Per  cent  moisture  based  on 
water-free  soil 
Before  irrigating     After  irrigating 

North  side 

6  in. 

.5.70 

6.38 

North  side 

20  in. 

5.04 

6.95 

East  side 

6  in. 

6.72 

12.61 

East  side 

20  in. 

7.99 

10.25 

South  side 

6  in. 

.5.70 

6.04 

South  side 

20  in. 

7.87 

7.29 

West  side 

6  in. 

7.52 

11.48 

West  side 

20  in. 

7.06 

11.60 

A  practical  horticulturist  after  examining  the  trees  and  digging  into 
the  soil  would  hardly  CDnelude  that  the  trees  were  suffering  for  w-ater. 
Moreover,  Portier  states"  that  in  sandy  loam  soils  6  per  cent  by  weight 
of  free  water  is  sufficient  to  keep  citrus  trees  in  a  vigorous  condition. 
In  the  Riverside-Redlands  di.striets  the  average  moisture  content  of 
the  .soils  in  citrus  orchards  runs  from  4  to  9  per  cent,  depending  on  the 
soil  type.  In  spite  of  this  it  is  possible,  of  course,  that  the  average 
moisture  content  of  the  Edison  soil  is  below  the  optimum. 

The  management  of  the  orchard  consists  of  clean  shallow  cultivation 
throughout  the  year  with  a  fairly  deep  plowing  in  I\rarch.  No  cover 
croi).s  have  as  yet  been  grown.    Light  applications  of  manure  and  com- 


;  Irrigation  of  Orchards,  TJ.  S.  Dept.  Agr.  Farmers'  Bull.  no.  404  (1910)  p.  24. 


! 


1919]       Coit-Eodgson :  AhnonnaJ  Shedding  of  Washington  Navel  Orange  287 

mercial  fertilizers  are  given.  The  roots  of  the  trees  fully  oecu]iy  all 
of  the  middle  spaces,  and  appear  exceptionally  healthy  and  vigorous. 
A  large  number  of  healthy  roots  were  taken  from  a  hole  dug  at  the 
center  of  a  square  formed  by  four  trees.  The  vertical  distribution  of 
roots  is  good.  A  hole  two  feet  square  v.-as  dug  to  the  southwest  of  a 
tree  well  beyond  the  spread  of  the  branches.  Each  six-inch  soil  layer 
was  kept  .separate  and  the  roots  sifted  out.  On  account  of  the  dryness 
of  the  air  comparative  weights  were  not  made,  but  the  root  distribution 
between  the  second  and  sixth  six-ineh  layer  is  well  sluiwii  in  plate  42. 

The  general  health  and  appearance  of  tlie  trees  at  the  Kellogg  orchard 
is  in  every  way  similar  to  that  at  Edison.  The  orchard  is  one  year 
younger  than  the  plot  used  in  the  experimental  work  at  Edison,  but 
there  is  no  appreciable  difference  in  the  size  of  the  trees,  unless  it  be 
in  favor  of  the  trees  at  the  Kellogg  place,  which  is  to  be  explained 
as  due  to  the  method  of  handling  the  orchard. 

Soil  conditions  are  fairly  similar,  except  that  the  surface  soil  at 
Edison  is  considerably  heavier  and  more  compact  than  at  East  Baker.s- 
field,  where  the  soil  would  beclassified  as  a  medium  sand.  However, 
it  becomes  heavier  as  one  goes  down  until,  at  a  depth  of  two  feet,  there 
is  no  notiecable  difference  in  the  soil  at  the  two  .stations.  We  are  not 
able  to  present  analyses  of  this  soil  as  to  plant  food,  but  there  is  no 
reason  to  believe  that  it  differs  markedly  from  that  at  Edison. 

A  radical  difference,  however,  is  manifest  in  the  management  of 
the  two  orchards.  The  main  part  of  the  Kellogg  orchard  is  planted 
to  alfalfa  (pi.  26),  and  the  portions  in  which  our  experimental  work 
was  done  have  had  alfalfa  grown  between  the  trees  for  three  or  four 
years.  Before  planting  the  alfalfa  the  orchard  was  carefully  and 
effectively  laid  out  in  .small  checks  draining  one  into  the  other.  The 
trees  an;  protected  from  having  water  standing  about  their  trunks  by 
ridges  thrown  up  just  under  the  drip  of  the  trees.  These  checks  as 
well  as  the  ridges  are  occupied  by  a  good  stand  of  alfalfa,  which  is 
cut  for  hay  and  hauled  off.  Irrigation  water  is  pumped  from  wells 
and  is  applied  in  copious  amounts,  the  period  between  irrigations 
averaging  about  three  weeks,  or  a  week  to  ten  days  shorter  than  that 
at  Edison.  There  can  hardly  lie  any  doubt  but  that  considerably  more 
water  is  applied  to  these  trees  than  at  Edison.  Applications  of  com- 
mercial fertilizers  have  been  made  to  the  orchard  from  time  to  time. 
No  detailed  study  of  the  root  distribution  was  made  but  a  few  holes 
dug  for  othi'r  purposes  seemed  to  indicate  thai  Ihe  roots  tend  to  go 
down  or  away  from  the  .surface  in  this  orchard  rather  than  to  be 
localized  in  the  ujjper  soil  layers. 


288  University  of  California  Publications  in  Agricultural  Sciences      [Vol.  3 

Another  distinctive  feature  of  the  KeHogg  orchard  is  that  it  is 
protected  on  three  sides  hy  a  fairly  efficient  windbreak.  On  the  north, 
frojn  wliieh  direction  the  prevailing  winds  blow,  this  consists  of  a 
double  row  of  pepi)er  trees  {Schiuus  moUe).  and  a  single  row  of 
poplars.  On  the  other  two  protected  sides,  the  east  and  the  west,  there 
are  rows  of  eucalj'ptiis. 

The  Nature  of  June  Drop 

A  cursory  investigation  of  the  problem  at  once  established  the  fact 
that  the  young  oranges  are  shed  while  still  alive  and  actively  function- 
ing and  as  such  the  .shedding  constitutes  true  abscission.  It  is  of 
course  quite  a  different  process  from  exfoliation,  which  involves  the 
formation  and  activity  of  a  phellogen.  Before  proceeding  to  a  di.s- 
cussion  of  the  process  of  abscission  as  determined  by  us,  it  may  be 
well  to  discuss  the  amount  of  bloom,  time  of  abscission,  reaction  time, 
and  other  important  features. 

Navel  orange  trees  growing  imder  the  conditions  studied  always 
bloom  very  heavily  (pis.  27,  28.  and  29).  The  blossoms  are  borne  on 
shoots  of  the  current  sea.son's  growth,  being  preceded  and  aeeompanied 
by  new  leaves.  The  old  leaves  do  not  fall  until  anthesis  is  well  under 
w-ay  or  completed.  It  is  evident,  therefore,  that  during  anthesis  the 
trees  are  under  a  heavy  drain,  inasmuch  as  they  are  called  upon  to 
support  a  heavy  bloom  in  addition  to  both  the  new  and  old  crops  of 
leaves.  Shedding  of  the  unopened  flower  buds  occurs  to  a  small  extent 
only.  The  opened  flowers  exhibit  a  certain  amount  of  dimorphism. 
Those  capable  of  setting  fruit  po.sse.ss  large,  fully  formed  ovaries,  with 
plump  stj'les  and  stigmas.  In  many  of  the  flowers,  however,  the 
pistils  show  a  varying  degree  of  degeneration  and  shedding  of  the 
flowers  is  largely  confined  to  such  individuals,  beginning  with  the  least 
robust  and  grading  off  during  petal  fall  and  including  many  of  the 
most  robust  after  petal  fall.  The  period  of  maximum  shedding  takes 
place  when  the  young  fruits  are  from  one-half  to  two  centimeters  in 
diameter.  At  first  the  point  of  abscission  is  always  at  the  base  of  the 
pedicel  fpl.  30),  but  after  the  diameter  of  the  fruit  has  reached  one 
centimeter  or  thereabouts  it  is  usually  at  the  ba.se  of  the  ovary.  It  is 
interesting  to  note  that  where  the  larger  fruits  absciss  at  the  base  of 
the  ovary,  abscission  usually  occurs  also  in  the  cortex  at  the  ba.se  of 
the  pedicel ;  but  on  account  of  the  formation  of  strengthening  tissue 
the  process  is  not  completed  through  the  va.scular  elements  and  although 
the  pedicel  dies,  it  remains  very  firmly  attached  to  the  twig.  This  is 
shown  in  plate  31.    It  often  happens  that  a  certain  amount  of  strength- 


lOlSl]       CoH-Hodfison:  Abnormal  Shcchlinij  of  fl'ashington  Navel  Orange  289 

ening  tissue  at  the  base  of  the  ovary  may  prevent  the  fall  of  the  fruit. 
These  dead,  dry  fruits,  as  shown  in  plate  31.  are  often  quite  conspicuous 
on  the  trees.  Soon  after  the  application  of  the  stimulus,  but  several 
days  before  actual  separation.  tli(>  larger  fruits  assume  a  characteristic 
appearance,  lo.sint;  their  luster  ami  taking-  on  a  lighter  green  color.  Tn 
the  ease  of  exposed  fruits  the  yellow  color  is  deeper  around  the  apex,  but 
this  is  not  the  case  with  shaded  fruits.  It  is  thus  a  simple  matter  to 
select  any  number  of  fruits  which  are  destined  to  absciss  several  days 
before  separation  actually  occurs. 

Experinicnts  carried  on  in  the  laboratory  and  observations  made  in 
the  field,  both  in  a  survey  of  the  citrus  districts  t)f  southern  California 
immediately  following  the  heat  wave  of  Jiuie  15-17,  1917.  and  at 
Bakersfield  during  1916  and  1917,  have  shown  that  the  time  inter- 
vening between  the  application  of  the  stimulus  and  actual  separation 
is  from  four  to  ten  days.  The  shorter  periods  were  obtained  in  the 
laboratory,  whci-e  the  room  temperature  was  uniroi-mly  high.  0\u' 
observations  are  that  under  field  conditions  abscission  is  ordinarily 
complete  within  five  to  eight  days  after  the  application  of  tbe  stimulus. 

Normally,  orange  blossoms,  being  borne  in  cymes,  open  in  succession. 
beginning  about  JIareh  20  in  the  San  Joa(|uin  N'aili'y  and  i-untinuing 
about  one  month.  Al)seission  varies  with  the  sea.son  but  usually  it  is 
in  evidence  from  April  1  to  about  July  1.  a  period  of  three  months. 
The  period  of  maximum  shedding  occurs  during  the  latter  half  of 
April.  It  should  be  noted  that  the  season  <if  1917  was  uni(|uc  in  being 
the  latest  on  record.  Protracted  cool  weather  delayed  the  bloom  fully 
five  weeks,  with  a  conseciucnt  delay  of  the  period  of  maximum  shedding. 
A  comparison  of  the  mean  maximum  atmospheric  temperatures  for  the 
years  1914-17  inclusive  is  shown  in  table  2.  The  comparative  lateness 
of  the  lf)17  season  is  app;irent  fnnii  a  stu<ly  of  this  table. 

TABLE  2 

Monthly  Me.\n  .M.vxi.mum  Temperatures  for  Tex  Months  at  Bakersfield 

Compiled  from  V.  S.  Weather  Bureau  Records 


11)14 

ini.5 

1916 

1917 

January 

61.2 

60.3 

57.8 

59.3 

Fcliruary 

67.9 

6.5.2 

69.9 

68.2 

Marcl. 

7o.8 

71.8 

73.8 

69.5 

A|.ril 

IHS) 

75.3 

81.6 

74.7 

May 

86.6 

77..5 

81.1 

77.4 

June 

94.7 

92.8 

93.0 

95.6 

July 

100.2 

98.8 

99.0 

104.4 

August 

:o:.9 

101  ..5 

95.2 

100.5 

September 

88.0 

91.9 

93.2 

94.3 

October 

82.9 

87.8 

76.1 

88.5 

290  University  of  California  Publications  in  Agricultural  Sciences      [Vol.  3 

Turning  now  to  a  more  detailed  account  of  the  abscission  process 
itself  we  find  that  this  subject  has  received  considerable  study  and 
investigation.  The  nature  of  the  abscission  proce.ss  has  been  studied 
and  described  in  detail  by  Hannig"  and  Lloyd^  for  Mirabilis:  Balls'* 
and  Lloyd"  for  Gossijpiit»i ;  Loewi"  for  Amprlopsis;  Kubart'  for 
Syringa  and  Nicoiiana;  Kendall"  for  Xirotiana;  Tison"  and  Lee"  for 
many  other  plants,  to  mention  only  a  few  of  the  researches  in  this 
interesting  field.  AVhile  the  histology  of  abscission  in  Citrus  has  been 
described  in  detail  elsewhere'-  by  the  junior  author,  it  is  appropriate 
that  a  brief  sketch  be  included  here. 

As  previously  indicated,  there  are  two  entirely  distinct  abscission 
zones.  One  is  at  the  base  of  the  pedicel  and  the  other  at  the  base  of 
the  ovary.  In  each  case  the  zone  may  be  considered  to  be  situated  at 
the  base  of  an  internode  where,  on  account  of  the  power  of  forming 
adventitious  buds,  it  may  reasonably  be  suspected  that  the  tissue 
retains,  to  a  degree  at  least,  its  nieristematie  nature.  The  zones  con- 
sist of  ten  to  eighteen  layers  of  cells  which  in  young  tis.sue  differ  his- 
tologically verj'  little,  if  any.  from  adjacent  tissues.  In  older  material 
differences  involving  shape,  size,  and  content  appear.  That  in  the  case 
of  young  tissue  differences  of  some  kind  do  exist  is  shown  by  the  fact 
that  after  the  stimulus  has  been  applied,  yet  ten  to  fifteen  hours  before 
visible  indications  appear,  the  walls  of  abscission  cells  are  differentiated 
by  a  marked  inability  to  hold  certain  stains,  such  as  raethylen  blue. 
From  six  to  eight  hours  before  abscis-sion  the  walls  of  the  abscission 
cells  are  refractive  to  a  different  degree. 

The  first  indication  of  actual  abscission  is  a  marked  swelling  and 
and  gelatinization  of  tlie  walls,  which  mav  amount  to  as  much  a.s  200 


3  Untersuchungen  iilier  das  Abstossen  von  Bliiten,  Zeitschr.  f.  Bot.,  vol.  5 
(19:3),]).  417. 

*  Abscission  in  Mirabilis  Jalapa,  Bot.  Gaz.,  vol  61  (1916),  pp.  213-30,  jil.  13. 

5  The  Cotton  Plant  in  Egypt  (London,  Macmillan,  1912),  p.  69. 

«  The  Abscission  of  Flower-buds  and  Fruits  in  Gossypium,  and  its  Relation  to 
Environmental  Changes,  Trans.  Roy.  Soc.  Canada,  ser.  3,  vol.  10  (1916),  pp.  55-61. 

"  Bliittablosung  und  verwandte  Erscheinungen,  A'ienna  Acad.  Proc,  vol.  1 
(1907),  pp.  166-983;  S-B.  d.  math.-nat.  Kl.  d.  k.  Akad.  Wiss.,  Wien,  vol.  116, 
abt.  1  (1907),  pp.  983-1024. 

8  Die  organische  Abliisung  der  KoroUen  nebst  Bemerkungen  iiber  die  mohlsche 
Trennungschicht,  Ibid.,  vol.  115  (1906),  p.  1491. 

0  Abscission  of  Flowers  and  Fruits  in  the  Solonaceae  with  special  reference 
to  Nicotiana,  Univ.  Calif.  Publ.  Bot.,  vol.  5  (1918),  pp.  347-428. 

'0  Recherches  sur  la  chute  des  feuilles  chez  les  DicotylMones,  Mem.  Soc.  Linn. 
Normandie,  vol.  20  (1900),  p.  12.5. 

u  The  Morphology  of  Leaf  Fall,  Ann.  Bot.,  vol.  25  (1911),  pp.  51-106. 

12  An  Account  of  the  Mode  of  Foliar  Abscission  in  Citrus,  Univ.  Calif.  Publ. 
Bot.,  vol.  6  (1918),  pp.  417-28. 


1919]      Coit-Bodgson :  Abnormal  Shedding  of  Washington  Navel  Orange  291 

to  300  per  cent.  This  is  followed  by  dissolution  of  the  gelatinous 
walls,  thus  freeing  the  cells  which  are  now  surrounded  merely  by  the 
very  thin  and  delicate  tertiary  membrane.  No  elongation  of  the 
tertiarj'  membrane  has  been  observed.  Neither  has  anj-  cell  division 
prior  to  separation  been  seen  to  occur,  although  immediately  following 
separation  this  often  takes  place.  So  f|ir  as  ascertained,  therefore, 
ab.sei.ssion  in  the  orange  conforms  to  the  usual  type,  e.g..  schizolysis'' 
representing  di.s.solution  of  the  middle  lamellae  of  the  abscission  zone 
cells  by  hydrolysis  with  subsequent  separation. 

Stimuli  Leading  to  Abscission 

The  direct  cause  of  abscission  in  plants  in  general  is  considered  to 
be  some  stimulus  which  may  be  brcmght  into  play  in  a  variety  of  ways, 
depending  somewhat  on  the  nature  of  the  plant  involved.  Lloyd'*  has 
taken  pains  to  enumerate  some  of  the  different  kinds  of  stimuli  wliich 
according  to  various  writers  have  been  found  to  cause  abscission.  It 
is  our  purpose  to  consider  these  in  turn  as  a  possible  cause  of  abscission 
in  the  Navel  orange  anil  possibly  by  elimination  to  arrive  at  the  true 
cause  or  causes  involved. 


Mechanical  Shock  or  Traumatic  Stimuli 

Fitting"  has  shown  that  jarring  or  shaking  the  flower  stalks  of 
Verbascum  sp.  and  (rcraniiim  pyrenaicum  will  result  in  abscission 
within  a  few  minutes.  AVe  were  unable  to  produce  like  results  with 
Citrus  by  Ibis  method.  .Moreover,  abscission  has  been  observed  to 
occur  regularly  under  conditions  which  would  preclude  the  possibility 
of  this  cause  l)eing  operative  with  oranges. 

An  effort  was  made  to  eau.se  abscission  by  cutting  and  bruising  the 
young  fruits  in  various  ways.  The  result  was  a  failure  in  every  case. 
Exeis.sion  of  the  style  and  petals  either  separately  or  together,  either 
before  or  during  anthesis.  failed  to  i)roduce  abscission.  Many  of  the 
fruits  from  which  the  style  had  been  removed  developed  to  maturity 
in  a  normal  way.  Otiiers  abscissed  but  the  reaction  time  varied  so 
widely  as  to  make  it  very  improbable  that  tiie  removal  of  tlie  style 
was  the  stiimilus  involved. 


I'Correns,  Vermehruii^  der  Laubnioosc,  .Iciia,  1899.     (CiteJ  from  Lloyd.) 
n  Abscission,  Ottawa  Naturalist,  vol.  28  (1914),  pp.  41-52,  61-7.5. 
15  Untersuclninfjen  iiber  die  vorzeitige  Entbliitterun};  von  Bliiten,  .lahrb.,  f. 
Wiss.  Bot.,  vol.  4!)  (1911),  p.  187. 


f 


292  University  of  California  Publications  in  Agricultural  Sciences      [Vol.3 

Id  many  plants  insect  injuries  have  been  shown  In  he  the  cause  of 
abscission.  The  case  of  the  cotton  boll  weevil  is  perha])s  the  best  known 
example,  though  it  i.s  likely  that  young  fruits  of  the  plum  and  apple 
react  to  injuries  due  to  the  curculio'"  and  codling  moth''  in  much  the 
same  way.  In  view  of  these  observations  it  is  interesting  to  find  that 
oranges  are  a  marked  exception  to  the  rule,  the  young  fruits  being 
particularly  resistant  to  the  effects  of  in.sect  wounds.'*  In  the  San 
Joaquin  Valley  there  are  tw-o  insects  at  least  which  cause  serious  injurj' 
to  the  fruit.  The  work  of  Scirfofhrii)s  citri  results  in  an  extensive 
though  superficial  scarring  of  the  fruit,  yet  the  fruit  develops  to 
maturity.  The  nymphs  of  the  fork-tailed  katydid.  Scuddi  ria  furrafa, 
eat  holes  in  the  young  f raits  (see  pi.  32),  the  holes  sometimes  extend- 
ing entirely  through  the  orange.  This  insect  produces  traumatic 
stimuli  of  the  first  magnitude,  yet  they  do  not  result  in  abscission. 
Large  numbers  of  the  chewed,  deeply  scarred  and  distorted  fruits 
develop  to  maturit.y  only  to  be  discarded  by  the  pickers  at  harvest  time. 

Mechanical  shock  produced  by  transplanting  trees  or  the  root 
pruning  incident  to  heavy  spring  plowing,  such  as  is  necessary  to  turn 
mider  a  rank-growing  cover  crop,  is  usually  followed  by  more  or  less 
dropping  of  the  leaves  and  fruit.  It  is  believed,  however,  that  this 
may  be  accounted  for  liy  the  disturbance  of  the  water  relations  w  hich 
follows  root  pruning  rather  than  by  the  mechanical  shock  alone. 
Balls,"  by  root  pruning  cotton  plants  in  Egypt,  was  able  to  cause 
abscission  of  the  bolls,  which  he  explained  on  the  ground  of  water 
relations  rather  than  shock.  This  particular  phase  of  the  problem  \\iil 
be  again  referred  to  later. 

Air  Temper.\tures  and  Light  Changes 

Abnormally  high  air  temperatures  or  .sudden  changes  in  the  tem- 
perature are  by  .some  investigators  considered  the  cause  of  abscission 
in  certain  eases.  It  is  evident  that  the  (juestion  of  the  influence  of  air 
temperature  is  so  involved  with  other  important  questions,  such  as  the 
influence  of  humidity,  air  movement,  transpiring  power  and  the  like, 
that  it  is  inadvisable  to  a.ssign  specific  influences  to  this  factor  alone. 
The  same  is  true  of  changes  in  light  intensity.     Suffice  it  to  say,  how- 


i«The  Plum  Curculio,  U.  S.  Dept.  Agr.  Bur.  Ent.,  Circ.  73  (1906),  p.  4. 

1'  The  Codlinfr  Moth,  ibid..  Yearbook  (1887),  p.  90. 

18  True  in  California,  though  Huljbard  mentions  the  punctures  of  two  insects, 
Dy.idercus  suturcUux  and  Lcptoplossus  phi/llopus,  as  causing  the  dropping  of 
mature  oranges  in  Florida.  IIul>l)ard,  H.  S.,  Insects  Aflfecting  the  Orange,  U.  S. 
Dept.  Agr.,  Div.  Ent.  (1885),  pp.  167-69. 

i»  Loc.  cit.,  p.  68. 


1919]      Coit-Hodgson :  Abnormal  Shedding  of  Washington  Navel  Orange  293 

ever,  that  while  a  siuKlcu  rise  in  tt'iiipcraturc  may  be  and  often  is 
aecompaDied  l)y  increased  shedding  rates,  it  has  been  observed  by  the 
writers  that  pnifnse  sliedding  of  the  young  Navel  oranges  takes  place 
during  periods  wiieu  no  sudden  changes  or  abnormally  high  tempera- 
tures occur.  It  has  also  been  noted  that  abscission  of  the  interior 
and  well  shaded  fruits  takes  place  simultaneously  with  that  of  fully 
exposed  fruits.  It  is  altogether  unlikely,  therefore,  that  the  June  drop 
can  be  explained  on  these  grounds  alone.  The  relation  between 
abscission  and  tissue  temperatures  as  affected  by  water  deficits  will 
be  discu.ssed  in  another  place. 

Many  investigators  have  noted  the  marked  effect  of  increase  in  air 
teini)eratures  on  the  time  involved  in  the  separation  process,  and  we 
have  noted  the  same  phenomenon.  The  eifeet  of  course,  as  would  be 
expected,  is  an  acceleration  conditioned  by  the  magnitude  of  the  tem- 
perature change.  It  appears  therefore  to  the  writers  that  abscissiou 
following  sudden  increa.ses  in  temperature,  as  noted  by  several  investi- 
gators, may  be  easily  explainetl  on  the  gi'ound  that  the  stimulus  to 
abscission  had  been  activated  at  some  time  prior  to  the  sudden  change 
in  temperature,  and  the  acceleration  of  the  abscission  process,  produc- 
ing marked  results  in  a  comparatively  short  period,  has  led  them  to 
believe  that  the  change  in  teiii]ieratiire  is  the  causative  stimulus. 

L.VCK  OF  POI-LIN.VTION  .\ND  FeRTIL1Z.\TI0N 

While  there  is  a  general  rule  that  pollination  and  I'ertilization  is 
essential  to  the  setting  and  development  of  fruits,  the  rule  is  con- 
spicuous for  its  exceptions.  A  uuinbci-  of  our  commercially  important 
fruits,  such  as  biiuauas,  Sultanina  gi'apes,  Jaj)aiiese  persimmons,  and 
Navel  oranges,  arc  distinctly  parthcnocarpic  and  do  not  require  the 
stimulus  of  pollination  to  insure  the  setting  of  fruits  which  are  usuall.v 
seedless.  The  Navel  orange  does  not  produce  viable  pollen,  and  pollen 
from  other  varieties  will  only  occasionally  accomplish  fertilization  for 
the  reason  that  nearly  all  of  the  embryo  sacs  disintegrate  in.stead  of 
dcvcliipiiig  inio  normal  ovules  cajialilc  nl'  l)ciiig  I'crtilized.-"  Occasion- 
ally a  few  normal  embryo  sacs  may  be  produced  and  seeds  result  pro- 
vided the  particular  fruits  having  the  normal  embryo  sacs  happen  to 
be  pollinated  with  viable  pollen  from  congenial  varieties.  It  is  the 
remoteness  of  the  chance  of  this  oceiu'riiig  uudi'r  ordinary  field  con- 
ditions that  accounts  for  the  comparative  seedlessness  of  these  fruits. 
Apparently  there  is  nothing  in  the  structure  of  the  blossom  of  the 


2"  Ikeda,  T.,  On  the  Parthenocarpy  of  ritriis  Fruits.  .lour.  Sci.  Afir.  Soc.  Tokyo, 
vol.  63  (1904). 


294  University  of  California  Publications  in  Agricultural  Sciences      [Vol.3 

Navel  orange  wliich  would  interfei-e  with  the  germination  of  pollen  or 
the  normal  extension  of  the  pollen  tube.  The  exclusion  of  pollen  by 
the  bagging  method  ha.s  shown  that  in  setting  fruit  the  Navel  orange 
is  entii'cly  independent  of  pollen.  This  experimental  evidence  is  borne 
out  by  the  practical  experience  of  growers  wOio  secure  as  abundant 
crops  from  large  isolated  plantings  of  Navels  as  from  mixed  plantings. 
It  is  therefore  entirely  safe  to  conclude  that  lack  of  pollination  and 
fertilization  of  the  Navel  orange  does  not  resifit  in  the  stinnilus  leading 
to  abscission. 

Relative  Position  on  Stem 

There  is  some  variation  in  the  relation  borne  by  orange  fruits  to 
the  main  supporting  axis.  As  it  has  been  suggested  that  with  some 
other  plants  this  relation  largely  determines  whether  a  given  fruit 
will  be  able  to  persist,  it  was  thought  worth  w-hile  to  investigate  the 
importance  of  this  point  in  connection  with  oranges.  A  large  number 
of  fruits  were  examined  and  divided  into  two  classes:  those  which 
terminated  the  axis,  and  those  which  did  not.  These  two  classes  are 
well  illustrated  in  plate  33.  It  seems  reasonable  to  suppose  that  in  the 
ease  of  the  non-terminals,  an  organ  of  limited  secondary  thickening 
(the  pedicel)  being  in  competition  with  one  of  unlimited  secondary 
thickening  (the  main  axis)  might  suifer  from  an  increasing  prejudice 
to  its  water  supply.  It  was  foi;nd  by  counts  of  large  numbers  of 
fruits  that  the  ratio  of  terminals  to  non-terminals  was  5  to  6.  The 
new  current  season's  growth  which  bore  terminal  fruits  averaged  3.8 
leaves  per  shoot,  while  the  non-terminals  averaged  3.95  leaves  per 
shoot.  In  the  latter  ca.se  1.85  leaves  were  below  and  2.1  leaves  above 
the  fruits.  Counts  of  fruits  which  had  successfully  survived  the 
abscission  period  showed  on  one  tree  16  terminals  to  31  non-terminals, 
but  on  another  tree  25  terminals  to  14  non-terminals.  Counts  of 
dropped  fruits  also  failed  to  support  the  above  supposition,  and  it  is 
evident  from  our  examination  of  large  numbers  of  specimens  that 
abscission  in  tliis  case  is  quite  independent  of  such  differences  in  the 
relation  of  fruit  to  axis  as  is  shown  in  plate  33. 

The  G.\s  F.\ctor 
It  has  long  been  recognized  that  the  .subjection  of  certain  plants  to 
an  atmosphere  containing  traces  of  various  narcotic  or  poisonous  gases 
is  .sufficient  to  cause  abscission  of  leaves  and  other  plant  j)arts.  One 
of  the  first  indications  of  smelter  fume  injury  is  the  shedding  of  the 
leaves  of  certain  plants  due  to  the  presence  of  sulfur  dioxide,  which  is 


1919]       Coit-Eodgson:  Abnormal  Shedding  of  Washington  Navel  Orange  295 

a  combustion  product  in  the  reduction  of  sulfur-containing  ores.  G. 
J.  Pierce-'  has  shown  that  when  SO,  is  present  in  as  small  quantities 
as  three  to  five  parts  per  million  abscission  of  the  leaves  of  certain 
forest  plants  occurs.  Several  investigators  have  reported  abscission 
of  Hower.s  and  leaves  of  various  plants  when  subjected  to  minute  traces 
of  illuminating  gas,  ether,  chloroform,  ethylene,  and  other  poisonous 
gases.  Further,  two  investigators  have  reported--'  -^  abscission  of  the 
leaves  of  citrus  plants  when  snlj.jccted  to  an  atmosphere  containing 
traces  of  illuminating  ga.s.  We  have  obtained  similar  results  with 
potted  plants.  Within  four  days  after  subjection  to  illuminating  gas 
all  the  leaves  were  shed. 

The  exhaustive  work  of  L.  I.  Knight  and  W.  Crocker"*'  -^  on  the 
eflPects  of  illuminating  ga.s  and  smoke  upon  plants  has  shown  rather 
conclusively  that  the  response  is  hirgcly  if  not  entirely  due  to  the 
toxicitj'  of  the  ethylene  present.  It  has  been  shown  by  E.  i\I.  Harvey^" 
that  as  minute  traces  as  one  part  per  nullion  are  sufficient  to  cause 
marked  reactions  on  the  part  of  the  plant. 

Preliminary  experiments  carried  out  in  our  laboratories  with 
excised  citrus  shoots  subjected  to  various  gases,  including  illuminating 
gas,  have  indicated  that  under  such  conditi(ms  absci.ssion  is  not  appre- 
ciably accelerated  by  any  of  the  gases.  The  time  at  which  shedding  of 
the  leaves  took  place  was  approximately  the  same  in  ordinary  room 
atmosphere  as  in  varying  concentrations  of  illuminating  gas. 

Peiree-'  has  shown  that  one  of  the  effects  of  smelter  fumes  is  to 
cause  excessive  transpiration  from  certain  plant  parts  prior  to  their 
abscission.  This  is  accounted  for  by  the  decomposition  of  the 
chlorophyll  in  the  i;uai'(l  cells  of  the  stoiiiata.  resulting  in  decreased 
stomatal  regulation  of  traiisj)irati(in.  As  will  be  pointed  out  later, 
several  investigators  have  concluded  that  abnormal  water  loss  during 
a  part  of  the  day,  resulting  in  con.siderable  fluctuations  in  the  leaf 


2'  1.  A  Report  of  an  Investigation  conilutced  for  U.  S.  Department  of  Justice, 
191."?,  uniiublislied  manuscripts  in  the  hands  of  U.S.  Attorney  General.  2.  Report 
of  Selhy  Commission,  to  l'.  S.  Bureau  of  Jfines,  1913. 

2=  In  Citrus  limonia.  Slionnard,  F.,  Tlie  Effect  of  Illuminating  Gas  on  Trees, 
Yonkers,  N.  Y.,  Dept.  Pub.  Works  (1903),  p.  48. 

23  In  Citrus  decumana.  Doubt,  Sarah  S.,  The  Response  of  Plants  to  Illuminat- 
ing Gas,  Bot.  Gaz.,  vol.  G3  (1917),  pji.  207-24. 

2<  The  Effect  of  Illuminating  Gas  and  Ethvlone  upon  Flowering  Carnations, 
Bot.  Gaz.,  vol.  46  (190S),  pp.  2;'59-7G. 

af' Toxicity  of  Smoke,  ibid.,  vol.  ri5  (1913),  pp.  337-69. 

=».Sonie  Effects  of  Ethylene  on  Metabolism  of  Plants,  ibid.,  vol.  60  (191.')), 
pp.  193-214. 

27  Expert  testimonv  incorporated  in  Records  of  Federal  Court,  District  of 
Utah,  Salt  Lake  City.' 


I 


296  TJniversUy  of  California  Publications  in  Agrindtural  Sciences      [Vol.  3 

water  content,  is  suffit'ieut  in  certain  plants  to  cause  abscission.  Li 
the  light  of  these  observations  abseission  of  plant  parts  when  exposed 
to  smelter  fumes  is  explainable  purely  on  the  basis  of  abnormal  water 
relations. 

In  an  effort  to  ascertain  whether  in  the  case  of  illuminating  gas  any 
such  relation  holds  true,  we  have  made  a  careful  study  of  the  stomata 
of  citrus  leaves  and  have  to  report  that  at  an  early  period  in  the  life 
of  the  leaf  they  lose  their  power  of  functioning  and  remain  practically 
closed  thereafter.  This  is  significant  in  view  of  our  findings  mentioned 
above,  namely,  that  illuminating  ga.s  is  not  a  direct  .stimulus  to 
abscission  in  Citrus,  at  least  with  excised  shoots.  In  the  case  of  potted 
plants  it  seems  probable  that  it  works  in  an  indirect  manner  throuirh 
disturbances  in  the  physiological  balance.  In  connection  with  the 
question  of  the  effect  of  illuminating  gas  upon  the  chlorophyll  of  the 
guard  cells,  it  should  be  mentioned  that  H.  M.  Richards  and  D.  T. 
MacDougal-*  have  reported  that  chlorphyll  formation  is  greatly 
retarded  wlien  the  ])lant  is  subjected  to  an  atmosphere  containing 
traces  of  this  gas. 

The  fumigation  of  citrus  trees  with  hydrocyanic  acid  gas  for  the 
ccmtrol  of  scale  insects  is  practiced  quite  generally  and  with  marked 
success  in  California.  It  is  the  general  experience  that  under  certain 
conditions  heavy  dosages  of  this  gas  result  in  abscission  of  the  older 
leaves.^"  Researches  by  Osterhout^"  and  Moore  and  Willaman^'  have 
showii  that  when  subjected  to  traces  of  this  gas  the  permeability  of 
cytoplasmic  septa  is  markedly  altered,  causing  an  increased  los-s  of 
water.  In  the  light  of  these  observations  it  is  entirely  possible  to 
explain  dropping  of  citrus  leaves  due  to  fumigation  on  a  purely  water 
relation  basis. 

Fumigation  injury  to  the  blossoms  or  fruit,  whether  large  or  very 
small,  consists  of  pitting  and  burning  which  results  in  scars  on  the 
fruit.  Apjiarently  in  no  case  does  fumigation  of  young  Navel  oranges 
with  hydrocyanic  acid  gas  furnish  a  stimulus  to  abscission. 

The  whole  subject  of  the  effect  of  gases  in  causing  abscission  of 
plant  parts  is  in  a  very  unsatisfactory  state  at  the  present  time.  In 
view  of  the  mass  of  conflicting  data,  as  well  as  the  fact  that  abscission 


-8  Tlip  Influpnoe  of  Carlion  Monoxide  and  other  Gases  upon  Plants,  Bull.  Torr. 
Bot.  Club,  vol.  ;n  (1904),  pp.  .57-66. 

=9  Woodworth,  C.  W.,  and  others,  School  of  Fumigation,  Pomona,  California, 
pp.  162-64,  Aujiust,  191.5. 

3"  Siniihirity  in  the  Effects  of  Potassium  Cyanide  and  of  Ether,  Bot.  Gaz., 
vol.  63  (iniT),'].)).  77-80. 

31  Studies  in  Greenhouse  Fumigation  with  Uvdrocvanic  Acid:  Physiological 
Effects  on  the  Plant,  Jour.  Agr.  Res.,  vol.  11   (ini7),  pp.  319-38. 


1919]       Coit-Bodgson:  Abnormal  Slicildiiifi  of  U'lixliittfiton  Xaiel  Orange  297 

of  young  Navel  oranges  occurs  throughout  the  great  interior  valleys 
of  California  and  in  districts  very  remote  from  any  possible  source  of 
noxious  vapors,  there  is  little  possibility  that  the  gas  factor  can  be 
operative  in  the  case  under  consideration. 

Fungi  and  Bacteria  as  a  Cau^e  op  Abscission 

Although  the  belief  is  commonly  held  by  plant  pathologists  that 
fungus  para.sites  sometimes  cause  the  shedding  of  plant  parts,  the 
literature  on  this  phase  of  absci.ssion  is  very  meager.  Inoculations 
with  Bncfn-iuiii  citrarefaciens,  the  organism  causing  Citrus  Blast, 
carried  on  in  our  greenhouses  have  shown  that  when  the  organism  is 
inoculated  into  the  tip  of  the  young  leaf  the  latter  is  shed  within  a 
few  days.  Rolfs  has  reported  that  shedding  of  mature  oranges  fre- 
quently occurs  in  Florida,  due  to  the  conunon  wither  tip  fungus, 
CoUetoiruhum  fjhosporioidrs.  However,  we  are  concerned  here  with 
the  shedding  of  immature  fruits  and  it  is  by  no  means  clear  that  the 
process  resulting  in  shedding  is  the  same  in  both  cases. 

For  many  years  growers  of  Washington  Navel  oranges  have  ex- 
perienced I0.SSPS  from  a  black  rot  disease  of  the  fruit  which  manifests 
itself  as  a  stimulation  of  the  fruit,  causing  it  to  grow  to  an  extra  large 
size,  ripen  early  and  assume  a  deep  red  color,  with  a  certain  amount  of 
dropping.  This  disea.se  was  first  noted  by  N.  B.  Pierce^-  in  1892  and 
was  first  described  by  him  in  1902^^  a.s  "Black  Rot  of  the  Navel 
Orange"  caused  by  the  fungus  Alternaria  citri. 

The  fruit  is  infected  wlirii  quite  small,  prdhably  just  brl'ore  or 
soon  after  the  style  is  shed,  through  the  cracks  and  inijjerfections  in 
the  proliferations  of  the  navel  (pi.  34).  The  fungus  is  a  weak  parasite 
and  remains  quiescent,  or  nearly  so.  during  the  growing  period  of  the 
young  fruit,  at  which  time  the  fruit  is  more  or  less  re.sistant  to  the  en- 
eroachnientsof  parasites.  AVitli  tlie  decline  in  vigor  incident  to  api)roa('h- 
ing  maturity  the  fungus  becomes  more  active  and  exerts  a  stinudating 
influence  on  the  fruit,  causing  it  to  take  on  a  deep  reddish-yellow  color 
and  to  ripen  earlier  than  the  normal  fruit.  In  a  small  and  restricted 
area  the  cells  of  the  pulp  are  broken  down  aiul  become  a  nauseating 
ma.ss  of  black  fungus  mycelia  and  spores.  The  rind  is  left  uiiiii.jured 
until  the  disease  has  made  considerable  progress  within,  Imt  ultimately 
a  black  and  decayed  spot  appears  on  the  surface  near  the  navel  end. 
A  certain  pro]>ortion  of  the  infected  fruits  early  shows  a  yellow  sjiot 


3=  U.  S.  Dept.  Agr.  Yparl)ook  (1892),  p.  239. 
3sBot.  Gaz.,  vol.  33  (1902).,  pp.  234-3.5. 


298  Vniversity  of  California  Publications  in  Agricultural  Sciences      [Vol.  3 

about  the  navel  end  and  drops  from  the  tree  when  about  one  to  two 
inches  in  diameter,  or  even  larger.  The  remainder  persi.st  to  maturity, 
the  disease  eominir  into  evidence  at  picking  time,  in  transit,  in  storage, 
or  not  until  in  tlie  hands  of  the  consumer. 

Early  in  1916  our  attention  was  directed  to  the  fact  that  on  dissec- 
tion a  relatively  large  number  of  the  shed  fruits  and  fruits  about  to 
drop  were  found  to  have  a  discolored  area  under  the  navel  end.  In 
many  cases  a  dark  colored,  gummy  mass  was  present,  although  in  others 
the  tissue  immediately  under  the  navel  was  only  slightly  discolored 
(pi.  35).  In  some  fruits  there  was  no  evidence  of  any  such  spot  or 
area.  A  few  of  the  dropped  fruits  were  .sterilized  in  mercuric  chloride 
(1-1000)  and  placed  in  small  moist  chambers.  To  our  surprise  these 
cultures  showed  practically  100  per  cent  infection  with  an  Altcrnan'a. 
Other  cultures  were  made  with  the  same  results.  Therefore  we  con- 
cluded that  it  was  well  within  the  realm  of  possibility  that  the  June 
drop  was  due  to  the  same  fungus  causing  black  rot  and  decided  to 
investigate  the  matter  more  thoroughly. 

The  fruits  had  reached  a  size  of  one  or  two  centimeters  and  the 
blooming  period  was  entirely  over,  precluding  any  investigation  as  to 
the  source  and  manner  of  infection  in  1916.  Therefore  our  efforts  in  this 
direction  during  1916  were  confined  to  attempts  to  determine,  if  po.s- 
sible,  the  extent  of  the  infection.  Cultures  of  many  hundreds  of 
shed  fruits,  and  fruits  about  to  fall,  from  many  districts  of  the  state 
were  made  both  by  the  method  above  described  and  by  inserting  a 
piece  of  tissue  from  the  discolored  area  into  slanted  tubes  of  Shear's 
corn  meal  agar.  The  cultures  uniformly  showed  a  high  percentage  of 
infection  with  Alfernaria.  A  few  cultures  were  then  made  using 
healthy  green  fruits  picked  from  the  trees.  The  percentage  of  infec- 
tion was  small.  Still  later  in  the  season  dropped  fruits  from  four  to 
five  centimeters  in  diameter  (pi.  35)  were  collected  from  districts  as 
far  apart  as  Oroville  in  the  Sacramento  Valley  and  El  Cajon  near 
San  Diego.  Cultures  made  from  tliese  fruits  showed  practically  TOO 
per  cent  infection. 

Although  the  uumln'r  of  cultures  made  was  too  small  to  justify 
a  broad  generalization,  the  work  done  in  1916  was  .sufficiently 
productive  to  form  the  basis  for  a  working  hypothesis  which  was 
advanced  as  a  theory  to  account  for  the  June  drop  of  Washington 
Navel  oranges.  Other  experimental  work  under  way  had  indicated 
the  presence  of  certain  abnormal  water  relations  between  the  young 
fruits  and  the  leaves  immediately  behind  them,  which   phenomenon 


1919]       Coit-Hodgson  :  Abnormal  Shedding  of  Ifashingtoii  Naicl  Orange  299 

will  be  discussed  more  fully  in  a  later  section.  Briefly,  the  theory 
advanced  was  that  excessive  transpiration  from  the  leaves  caused 
water  together  with  enzymatic  solutions  secreted  by  the  fungus  in  the 
navel  end  to  be  drawn  back  through  the  vascular  system  of  the  young 
fruits  through  the  pedicel  and  thus  provide  the  stimulus  to  abscission.'* 
That  there  is  no  mechanical  difficulty  involved  in  this  theory  was 
borne  out  when  by  means  of  dyestuff  solutions  it  was  demonstrated 
that  the  vascular  system  running  to  the  navel  or  secondary'  oi'ange 
travei*ses  the  central  pith  or  core  of  the  priiuar\'  fruit,  whicli  thus 
serves  as  receptacle  and  stem  to  the  smaller  fruit  (fig.  1). 


Pifr.  1.     Structure  of  the  Navel  orange.     The  central  jiith  containing  fibro- 
vascuiar  bundles  acts  as  the  stem  of  secondary  fruit. 

Further  evidence  tending  to  support  this  theory  lies  in  the  fact 
that  black  rot  is  much  more  prevalent  in  the  interior  valleys  than  in 
the  coast  regions.  In  fact,  there  seems  to  be  a  certain  correlation 
between  the  amount  of  black  rot  and  the  amount  of  drop.  The  reason 
for  the  greater  prevalence  of  black  rot  in  the  hotter,  more  arid  districts 
was  not  uncovered  until  later;  this  will  be  brought  out  i!i  another 
section. 


Alternaria  cilri,  Ellis  and  Pierce 

During  the   winter  of   1916   a  careful  study   of  the   alternarias 

obtained  in  our  cultures  was  made  and  disclosed  the  fact  tluit  although 

there  were  several  strains  of  Alternaria  obtained,  one  particular  type 

rather  easily  recognizable  after  a  little  practice,  was  by  far  the  most 


a<  Colt,  J.  Eliot,  and  Hodgson,  H.  W.,  The  f'avise  of  June  Drop  of  Washington 
Navel  Oranges,  Univ.  Calif.  Jour.  Agr.,  vol.  4  (1916),  p.  10. 


300  University  of  California  Puhli<^ations  in  Agricultural  Sciences      [Vol.  3 

common.  In  addition  we  obtained  one  strain  possessing  the  ascigerons 
stage,  which  of  course  classified  it  in  the  genus  Pleospora.  Several 
Macrosporium  strains  were  also  Lsolated. 

Considerable  effort  was  made  to  identify  the  Alternaria  strain 
so  commonly  found,  but  we  have  been  unable  to  satisfy  our.selves 
thoroughly  in  this  regard.  While  the  literature  is  indeed  voluminous, 
there  is  apparently  no  reliable  monograph  of  the  genus.  Recently, 
however,  there  has  appeared  a  critical  study  of  the  taxonomic  char- 
acters of  the  genus. "^  The  genus  Alternaria  is  one  of  the  most  uni- 
versally distributed  of  the  common  forms  of  the  Fungi  Impcrfecti. 
It  embraces  about  fifty  species,  although  it  has  been  shown  by  Elliott 
that  a  large  number  of  the  species  of  the  closely  related  genus  Macro- 
sporium really  belongs  to  the  genus  Alternaria.  Among  these  species  we 
find  active  para.sites  as  A.  solani  (E.  and  SI.)  J.  and  G.,  weak  or  facul- 
tative parasites  as  A.  citri  Ellis  and  Pierce,  and  saprophytes  as  A. 
tenuis  Nees.  Certain  species  have  already  been  secured  in  the  perfect 
or  ascigerons  stage  which  has  always  proved  to  be  Pleospora.  Since 
the  strain  under  consideration  was  uniformly  obtained  from  oranges  in 
a  district  where  black  rot  is  common  it  is  probably  the  same  form  found 
by  Pierce  and  called  Alternaria  citri.  We  were  unable  to  find  the 
original  description  by  him,  which  does  not  seem  to  have  been  pub- 
lished. However,  after  examining  the  literature  and  drawings  of 
Alternaria  citri,  particularly  as  given  by  Rudolph.^"  we  feel  reasonably 
sure  that  we  are  dealing  with  Alternaria  citri  E.  and  P.  and  throughout 
the  remainder  of  the  discussion  we  shall  proceed  on  that  assumption. 

The  spores  of  Alternaria  citri  are  borne  in  long  chains  (pi.  36), 
which  readily  break  up,  allowing  the  spores  to  float  away  in  the  air. 
It  seemed  important  to  determine  whether  the  infection  of  oranges  was 
accomplished  by  spores  borne  by  the  air  or  those  carried  by  honey- 
bees and  other  insects.  The  following  methods  were  employed.  Petri 
dishes  containing  Shear's  corn  meal  agar  were  exposed  for  five  minutes 
in  different  localities.  After  a  few  days  had  elapsed  in  order  to  allow 
the  various  bacteria,  molds  and  other  fungi  to  assume  colony  form 
and  the  Alternaria.  if  present,  to  produce  spores,  the  dishes  being 
inverted  were  placed  under  the  low  power  of  the  microscope  and  the 
colonies  of  Alternaria  easily  distinguished  and  counted.  On  account 
of  the  length  of  the  spore  chains  and   certain  other  morphological 


3=  Elliott,  J.  A.,  Taxonomic  Characters  of  the  Genera  Alternaria  and  Macro- 
sporum,  Am.  Jour.  Bot.,  vol.  4  (1917),  pp.  439-76. 

38  A  New  Leaf -Spot  Disease  of  Cherries,  Phytopathology,  vol.  7  (1917),  pp. 
188-97. 


1919]      Coit-Hodgson :  Abnormal  Shedding  of  Washington  Navel  Orange  301 

characters  which  became  familiar  wnth  practice,  it  was  easy  to  dis- 
tinguish between  various  other  species  of  Alternaria  which  were 
occasionally  met  with. 

The  specialized  cells  lining  the  stylar  canal  of  orange  flowers  secrete 
a  pure  white  sugary  mucilage  which  is  exuded  upon  the  stigma  in  a 
rather  large  drop.  This  material  is  an  excellent  medium  for  the  growth 
and  spornlation  of  Alternaria,  as  was  determined  by  trial,  the  fungus 
fruiting  heavily  in  a  short  time  on  smears  kept  in  a  moist  chamber. 
In  order  to  determine  the  amount  of  infection  of  blossoms  in  the 
orchard,  the  stigmas  were  clipped  with  sterile  scissors  on  agar  plates 
and  the  resulting  growths  examined  a  few  days  later  for  the  charac- 
teristic spore  chains.  The  data  secured  in  this  way  are  presented  in 
tables  3  and  4.  In  the  interior  valleys  89  per  cent  of  the  stigmas  were 
infected  and  in  coa.st  localities  76  per  cent.  It  is  found  that  the  air 
generally  throughout  the  state  carries  Alternaria  spores  in  abundance. 
In  interior  localities  Alternaria  spores  were  taken  in  78  per  cent  of 
exposures  with  ten  centimeter  agar  plates ;  in  some  places  near  the  coast 
in  63  per  cent.  It  was  also  shown  that  while  bees  may  and  do  carry 
spores  from  one  blossoni  to  another  the  number  of  spores  in  the  air  is 
sufficient  to  cause  widespread  infection  without  the  aid  of  bees. 


TABLE  3 

Nutrient  Agab  Plates  Exposed 

TO  THE  Air, 

1917 

Alternaria 

Alternaria 

Locality 

Date 

present 

not  present 

Whittier 

May 

."? 

18 

3 

Hijthlanil 

May 

6 

10 

0 

Edison  (under  tent) 

May 

3 

1 

2 

Oroville 

May 

14 

10 

2 

Berkeley- 

May 

17 

0 

4 

Fresno 

May 

28 

2 

0 

San  Leandro 

June 

3 

1 

4 

Fair  Oaks 

June 

12 

5 

0 

Edison  (orchard) 

June 

22 

1 

2 

Edison  (desert) 

June 

22 

3 

2 

Corona 

June 

27 

3 

0 

Whittier 

June 

26 

2 

1 

Riverside 

June 

29 

fi 

3 

Berkeley 

Oct. 

25 

1 

5 

In  this  coiini-ction  the  question  naturally  arises  as  to  why.  if  the 
infection  of  the  stigma.s  near  the  coast  is  a.s  great  as  76  per  cent,  there 
is  such  a  relatively  sniail  nuiiibiT  of  black  rot  oranges.  The  reason 
apparently  lies  in  the  fact  that  the  average  configuratidii  of  Hie  navels 


302 


University  of  California  Publications  in  Agriculttiral  Sciences      [Vol.  3 


is  more  irregular,  jagged  and  rough  (pis.  :34  and  37)  in  the  interior 
valleys  than  in  the  coast  districts,  where  tlie  navel  formation  is  nmch 
more  commonly  smooth  or  submerged  and  closed.  This  imperfect  and 
open  condition  of  the  navels  in  the  interior  valleys,  as  will  be  brought 
out  later,  is  due  to  the  harsher  environmental  complex  to  which  the 
fruits  are  subjected  during  the  growing  period.  Everyone  is  familiar 
with  the  fact  that  fruits  borne  in  exposed  positions,  particularly  in 
the  top  of  the  tree,  are  very  apt  to  be  coarse  and  rough  with  large 
protruding  navels,  while  the  interior  fruit  is  much  finer  in  texture. 
The  prevalence  of  Alternaria  spores  in  the  coa.st  districts  is  certainly 
not  much  less  than  in  the  interior  valleys,  but  the  amount  of  infection 
is  much  less  because  of  the  smaller  number  of  imperfect  navels. 


TABLE  4 

Miscellaneous  Cultubes 

Ait  erti  aria 

Aifernaria 

Locality 

Date 

Kind  of  material 

present 

not  present 

Whittier 

May    3 

Navel  blossoms 

10 

0 

Whittier 

May    3 

Valencia  blossoms 

2 

2 

Highland 

May    6 

Navel  blossoms 

14 

0 

San  .lose 

May  25 

Blossoms 

4 

1 

Oroville 

May  14 

Olive  blossoms 

1 

0 

Oroville 

May  14 

Navel  styles 

8 

0 

Oroville 

May  14 

Bees  about  trees 

3 

1 

Oroville 

May  14 

Lady  bird  {Vedalia  sp.) 

1 

0 

Berkeley- 

May  17 

Dead  style  from  greenhouse 

0 

1 

Berkeley 

May  17 

Dead  twig  from  greerihouse 

0 

1 

Fresno 

May  28 

Orange  blossoms 

5 

0 

Sacramento 

]\ray  25 

Orange  blossoms 

5 

0 

Fair  Oaks 

June  12 

Orange  blossoms 

9 

0 

Edison 

Apr.  24 

Orange  blossoms 

11 

1 

Edison 

Apr.  25 

Bees  about  trees 

3 

3 

Riverside 

.Tnne  27 

Citron  styles 

2 

0 

Edison 

May    2 

Navel  blossoms 

4 

1 

Edison 

May    2 

Valencia  blossoms 

4 

0 

Edison 

,May    2 

Pomelo  blossoms 

2 

0 

Edison 

May    2 

Soil  from  under  trees 

0 

2 

Edison 

May    2 

Bees  al)0ut  trees 

1 

1 

Edison 

May   2 

Navel  blossoms  from  tented  tree     6 

0 

As  is  shown  in  table  4,  Alternaria  spores  are  present  on  almost  all 
the  styles,  botli  in  coast  and  in  interior  valley  districts.  In  order  to 
ascertain  whether  infection  occurred  by  the  fungus  growing  down 
through  the  style  into  the  navel  end,  material  known  to  be  infected 
with  Alternaria  was  put  up  in  paraffine,  sectioned,  and  stained. 
Although  the  fungus  was  conspicuous  on  the  stigmatie  surface  no  traces 
of  fungus  mycelium  could  be  found  in  the  stylar  tissues.  This  fact, 
together  with  the  fact  tliat  infection  is  definitely  correlated  with  tlie 


1919]       Coit-llodyson :  Abnormal  Shedding  of  Washington  Diavcl  Orange  303 

configuration  of  the  navel  end,  renders  it  reasonalily  eertain  that 
infection  occurs  some  time  after  the  style  has  been  shed.  The  spores 
are  probably  blown  and  find  lodgment  in  ragged  open  navels  where 
they  arc  lield  in  tlie  crevices  till  enfolded  and  overgrown  by  the  rapidly 
developing  ovary  (pis.  34.  37).  Ina.smuch  as  the  configuration  of  the 
navel  as  well  as  its  size  and  degree  of  insertion  are  exceedingly  variable, 
it  is  evident  that  only  in  a  comparatively'  small  and  variable  number 
of  cases  are  the  spores  or  mycelium  so  situated  as  to  permit  germina- 
tion or  growth.  Alirrnana  citri  is  a  weak  para.site  and  cannot  pene- 
trate the  unbrolicn  skin  of  an  orange.  "While  it  is  not  capable  of  pro- 
ducing any  widespread  breakdown  in  the  tissues  of  immature  oranges, 
it  is  able,  after  introduction  into  the  fruit,  to  bring  about  a  certain 
stimulus  or  irritation  which,  according  to  our  theory,  results  in  abscis- 
sion of  a  eertain  iiroportion  of  the  young  fruits.  It  is  certain  that  as 
the  fruits  grow  and  approach  maturity  tlie  abnormal  size,  jireniature 
ripening,  and  extra  deep  color  are  the  direct  results  of  this  stimulation. 
It  is  also  considered  highly  probable  that  a  certain  proportion  of  the 
.splitting  or  deliiscence  of  the  carpels  which  is  .so  serious  in  interior 
vaUeys  is  connected  with  the  stimulation  of  these  infections. 

Referring  again  to  the  wide  distribution  and  general  prevalence  of 
Allertmria  spores  in  the  air,  it  is  evident  that  the  spores  may  be  trans- 
ported in  large  numbers  for  great  distances.  The  source  of  infection 
is  by  no  means  limited  to  the  vicinity  of  orchards.  The  fungus  grows 
readily  as  a  sapliropliyte  on  dead  leaves,  weeds,  twigs,  and  other  plant 
debris  and  it  is  entirely  po.ssible  for  spores  to  be  brought  in  from 
forest  areas  in  the  mountains  many  miles  away.  Spores  have  been 
taken  in  the  desert  far  from  cultivated  crops.  In  the  dry  aii-  of  the 
San  Joaipiin  Valley  the  black  rot  oranges  which  fall  under  the  trees 
are  not  immediately  decomposed  by  I'cnicillia,  Fusaria,  and  other  fungi. 
They  tend  to  munmiify  and  after  the  Alternaria  spreads  through  the 
interior  it  comes  to  the  surface,  and  the  spores  there  formed  give  these 
Diunnnies  a  black  color,  a.s  shown  in  plate  38.  These  nuimmics.  together 
«ith  the  large  number  of  aliscissed  styles  from  the  blossoms,  undoubt- 
edly furnish  a  gi-eally  increased  sui)ply  of  spores  at  the  critical  time 
in  the  development  of  the  fruit. 

A  rot  of  apples  occurring  in  Colorado'^  has  been  described  as  caused 
by  an  undetermined  species  of  Allcrnaria.  Judging  from  the  draw- 
ings presented  in  plate  4  of  Longyear's  iiublication.  the  fungus  is  very 
similar  to  if  imt  tlu'  same  as  that  with  wliii'h  we  are  dealing.    ^Moreover, 


•■"  Longyear,  B.  O.,  A  Xew  Apple   Hot,  Colorado  Apr.  Exp.  St.n.   Bull.   10.5, 
1905. 


304  University  of  California  Puhlications  in  Agricultural  Sciences      [Vol.  3 

there  is  a  marked  similaritj'  between  the  modes  of  infection.  Accord- 
ing to  Longj-ear  (p.  7)  : 

The  reason  why  certain  varieties  of  the  apple  are  particularly  subject  to 
the  blackened  seed  cavity  is  found  in  a  structural  peculiarity  of  such  varieties. 
Thus  a  longitudinal  section  through  such  an  apple  usually  shows  a  very  deep 
calj'x  tube,  which,  in  many  cases,  extends  to  or  meets  the  core,  or  even  opens 
into  it.  In  such  cases  the  fungus  has  evidently  reached  the  core  through  this 
passageway  by  following  the  united  styles  and  the  inner  wall  of  the  calyx  tube. 
(Italics  ours.) 

Only  certain  varieties  of  apples,  such  as  the  Winesap,  Ben  Davis  and 
a  few  others  which  have  the  structural  peculiarities  above  mentioned, 
are  found  to  be  affected  and  in  this  connection  Longyear's  remarks  on 
page  12  are  of  particular  interest  to  us. 

Some  of  these  varieties  are  among  those  which  are  reported  as  dropping 
their  fruit  badly  in  some  seasons  during  June  and  July,  but  whether  or  not  the 
fungus  plays  any  part  in  this  matter  has  not  been  determined. 

The  experimental  work  with  Alternaria  in  1917  for  several  reasons 
gave  quite  different  results  from  those  obtained  during  the  previous 
season.  As  is  shown  in  tables  3  and  4,  cultures  made  from  stigmas 
early  in  the  season  showed  a  high  per  cent  of  Alternaria  infection. 
However,  a  very  large  series  of  cultures  made  somewhat  later  in  the 
season,  from  the  young  fruits  one-half  to  two  centimeters  in  diameter, 
to  our  astonishment  showed  a  very  small  per  cent  of  infection.  Culture 
after  culture  showed  no  Alternaria  at  all.  Somewhat  later,  when  the 
fruits  were  larger,  cultures  of  the  shed  fruits  showed  a  higher  per  cent 
of  infection,  while  a  few  cultures  made  when  the  dropped  fruits  were 
four  to  five  centimeters  in  diameter  showed  a  high  per  cent  of  Alter- 
naria infection. 

Inasnutch  as  by  far  the  greater  part  of  the  drop  occurs  wliile  the 
fruits  are  one-half  to  two  centimeters  in  diameter,  at  which  time  our 
cultures  showed  comparatively  little  Alternaria  infection,  it  is  evident 
that  the  shedding  of  this  part  of  the  crop  can  not  be  attributed  to 
Alternaria.  However,  it  is  to  be  noted  that,  as  was  the  case  in  1916, 
toward  the  end  of  the  period  of  shedding  the  dropped  fruits  .showed 
a  steady  increase  in  tlie  per  cent -of  infection.  Evidently,  then,  the 
shedding  may  be  divided  into  two  parts,  the  first  including  small  fruits 
which  >nai/  or  may  not  be  infected  with  Alternaria,  the  second  includ- 
ing larger  fruits  which  are  infected  with  Alternaria. 

Inasmuch  as  the  climatic  conditions  in  the  San  Joaquin  Valley 
during  the  1917  season  were  considerably  more  severe  than  in  1916 


1919]       Coit-IIodgson :  Abnormal  Shedding  of  Washington  Navel  Orange  305 

(fig.  6),  and  therefore  the  average  configuration  of  the  navels  more 
ragged  and  open,  to  wliat  can  wo  attribute  tliis  difference  in  the  amount 
of  infection  with  Altcrnaria'  "We  believe  that  this  difference  is  easily 
explained  by  a  study  of  the  mean  maximum  temperatures  for  the  two 
seasons.  In  table  2  these  are  showTi  for  the  years  1914-17  inclusive. 
For  the  1917  sea.son.  taking  the  months  of  January  and  February,  we 
see  tiiat  they  are  about  average  for  the  last  four  years.  March  is  four 
or  five  degrees  below  the  average,  April  .still  more,  and  even  May  is 
below  the  average.  June  is  several  degrees  above  the  average  for  the 
last  four  years  and  July  shows  an  average  mean  maximum  temperature 
of  104.4°  F,  considerably  above  the  avei-age.  In  other  words,  the 
early  part  of  the  season  was  cooler  than  usual  and  the  bloom  was 
delayed  a  month  or  more.  Coincident  with  the  end  of  the  blooming 
period  the  weather  changed  radically  and  became  very  hot  and  dry  and 
continued  so  for  at  least  three  months.  Conditions  were  unfavorable 
for  infection  by  Alternaria ;  its  growth  wa.s  inhibited  although  the 
spores  were  ])rcseiit.  In  fact,  the  amount  of  drop  due  to  Alternaria 
in  1917  is  practically  negligible,  and  this  is  supported  b,y  the  fact  that 
there  were  very  few  black  rot  oranges  at  Edison  at  harvest  time.  On 
the  other  hand,  the  season  of  1916  was  noted  as  a  relatively  cool, 
pleasant  summer  and  as  such  was  favorable  for  infection  by  Altiriiaria, 
with  the  I'e.sult  that  there  were  many  black  rot  oranges. 

In  this  connection  the  ([uestion  arises,  why  are  not  other  citrus 
varieties  grown  in  these  arid  districts  subject  to  infection  by  Alternaria 
with  a  consequent  shedding  and  lo.ss  due  to  black  rot?  The  answer 
apparently  lies  in  two  fticts:  that  other  varieties  are  not  so  susceptible 
to  shedding,  which  will  be  discussed  later,  nor  are  they  mor|)li(ilogically 
adapted  to  infection  by  the  fungus.  Plate  39  shows  the  apical  end  of 
a  small  Valencia  orange  highly  magnified  and  it  is  evident  that  there 
is  no  favorable  entrance  for  the  fungus  spores.  Plate  34  shows  a 
similar  view  of  a  Navel  orange  with  very  favorable  conditions  for  the 
lodgment  of  fungus  spores. 

During  the  course  of  these  investigations  a  great  deal  of  time  and 
effort  was  devoted  to  attempts  to  ascertain  by  inoculation  methods 
whether  the  stimulus  of  Alternaria  citri  which  iiiiiiiifisted  itself  so 
clearly  in  the  change  of  color  of  the  fruit  might  not  also  be  the  eau.se 
of  abscission  of  the  young  fruits.  On  account  of  several  peculiar 
difficulties  inherent  in  this  particular  problem  we  have  .so  far  been 
unable  to  secure  conclusive  results.  The  three  most  iiii|)oi'1iiiil  of  these 
difficulties  mav  be  mentioned  brieflv  as  follows: 


306  Vniversity  of  California  Tuhlicaiions  in  Agricultural  Sciences      [Vol.  3 

1.  Referring  again  to  plate  29,  it  is  apparent  that  the  excessive 
number  of  buds  oecasions  a  severe  struggle  for  survival,  only  a  com- 
paratively small  number  being  able  to  acquire  water  and  food  sufticieut 
for  development.  As  it  is  impossible  to  determine  in  advance  which 
if  any  of  a  group  of  similar  buds  Ls  destined  to  remain,  it  is  evident 
that  if  the  sterile  .stigmas  of  all  are  inoculated  just  previous  to  open- 
ing many  will  eventually  fall  from  other  causes,  iloreover,  the  con- 
siderable period  of  time  involved  and  the  frequent  necessary  opening 
and  closing  of  the  bags  in  an  atmosphere  shown  to  be  filled  with  spores 
would  introduce  an  element  of  serious  error.  Plate  40  shows  one  of  a 
number  of  trees  u.sed  futilely  in  efforts  to  get  results  in  this  way. 

2.  Orange  flowers  are  dimorphic,  as  before  mentioned,  a  certain 
number  being  destined  to  fall  because  the  ovary  is  not  capable  of 
development.  The  configuration  of  the  navel  is  to  a  certain  extent 
fortuitous.  In  some  cases  the  epidermal  folds  are  so  adju.sted  a.s  to 
admit  infection,  in  others  not.  It  is  obviously  out  of  the  question  to 
examine  each  frviit  frequently  and  with  sufficient  minuteness  to  deter- 
mine whether  dui-ing  growth  an  opening  sufficient  for  the  entrance 
of  the  fungus  was  or  was  not  available. 

3.  A  species  of  aphis  is  very  common  on  Malva  and  other  weeds 
under  the  trees.  For  some  reason  not  at  present  clear,  the  insect  is 
unable  to  increase  to  any  extent  when  feeding  on  the  orange  leaves 
in  the  open.  However,  it  was  found  that  whenever  a  twig  was  enclosed 
in  H  paper  bag  or  a  tree  enclosed  in  a  cheesecloth  tent  (pi.  40)  the 
aphis  multiplied  at  an  astonishing  rate.  In  about  half  the  bags  on 
the  tree  .shown  the  twigs  were  defoliated  and  killed  by  the  sudden 
development  of  a  mass  of  aphis  from  young  and  minute  individuals 
which  were  inadvertently  included  within  the  bags  in  spite  of  all  pre- 
cautions. 

.Summing  up  the  relation  between  Alternaria  and  that  part  of  the 
June  drop  with  which  it  is  always  associated,  we  have  to  conclude  that 
inasmuch  as  the  presence  of  the  fungus  and  its  ability  to  provide  a 
certain  stimulus  have  been  demonstrated,  it  is  not  unreasonable  to 
suppose  that  abscission  may  be  another  manifestation  of  the  same 
stimulus  both  in  the  case  of  Navel  oranges  and  in  the  apple  varieties 
referred  to  above.  Satisfactory  scientific  evidence  of  this  point,  how- 
ever, is  lacking  as  yet. 


1919]       Coit-Hodgson :  Ab)wnnal  Shedding  of  H'ashington  Navel  Orange  307 

The  REii.\Tiox  of  Abscission  to  the  Environmental  Complex 

It  has  long  been  noted  that  there  exists  a  marked  correlation 
between  climatic  conditions  and  the  prevalence  and  amount  of  the 
June  drop.  This  correlation  has  been  discussed  somewhat  by  the  junior 
author  in  another  place^*  and  has  been  reflected  in  the  general  attitude 
of  growers  who  are  prone  to  assign  Juue  drop  to  hot  north  winds, 
sudden  changes  in  temperature,  and  other  causes,  most  of  which  are 
climatic  in  nature. 

In  order  to  ol)tain  more  accurate  information  in  this  i-egard  an 
investigation  of  the  yield  per  tree  in  different  citrus  districts,  where 
all  other  factors  except  the  climatic  complex  were  comparable,  was 
carried  out  during  the  season  of  1917.  The  results  were  striking  and 
show  mast  pronounced  correlation  between  climatic  conditions  and 
yield  wlien  all  other  factors  such  as  orchard  management,  etc.,  are 
fairly  comparable.  It  was  found  that,  assigning  a  yield  of  100  per  cent 
to  the  district  averaging  the  highest  crop,  which  district  is  character- 
ized by  considerable  summer  heat  but  moderate  atmospheric  humidity, 
the  farther  inlanil  the  district  lies  the  smaller  is  the  crop.  This  is 
precisely  the  order  in  wliich  the  asperity  of  the  environmental  cdin- 
plex  is  heightened,  the  atmospheric  humidity  decreasing  and  the 
average  summer  temperature  increasing.  ^loreover,  and  more  im- 
portant, distance  from  the  coast  brings  with  it  increasing  liability 
to  sudden  changes  in  the  weather  whicli  react  most  unfavorably  on 
crops,  particularly  when  in  certain  stages.  The  districts  where  these 
climatic  conditions  are  most  .severe,  namely,  the  Coachella  Valley  and 
the  southern  San  Joaquin  Valley,  show  a  yield  of  approximately 
25  per  cent  of  that  of  the  most  climatically  favored  district.  At 
intermediate  stations  the  extent  of  the  drop  and  consequently  the  size 
of  tlie  crop  is  easily  correlated  with  weather  conditions  during  the 
critical  period.  This  was  exemplified  bj'  the  almost  total  loss  of  the 
Navel  crop  in  tlie  district  between  Corona  and  Redlands  in  1017.  whrn 
a  dry  north  wind  of  unprecedented  severity  was  accompaiiictl  by  maxi- 
mum daily  temperatures  as  high  as  118°-120°  F  from  June  15  to  17. 

This  correlation  between  asperity  of  climatic  conditions  and  amount 
of  crop,  or  what  amounts  to  the  same  thing,  the  prevalence  of  dropping, 
was  very  apparent  in  the  orchard  where  our  experimental  work  was 
done  at  EdLson.    The  yield  from  the  particular  ten-acre  tract  used  was 


•■"s  TIo(]<»son,  Robert  W.,  Some  Abnormal  Water  Relations  in  Citrus  Trees  of 
the  Ariii  Southwest  and  their  Possible  Significance,  Univ.  Calif.  Publ.  Agr.  Sei., 
vol.  3  (1917),  i>p.  37-54. 


308  University  of  California  Publications  in  Agricultural  Sciences      [Vol.  3 

56  per  cent  less  in  1917  than  in  1916  though  the  trees  were  a  year  older 
and  should  have  yielded  more.  The  asperity  of  climatic  conditions 
during  the  critical  period  in  1917  as  integraded  in  the  Livingston  white 
porous  cup  atmometer  (pi.  41)  was  approxiiiiatel.y  40  per  cent  greater 
than  during  the  same  period  in  1916.  This  fact  is  brought  out  in 
table  5,  where  the  water  loss  from  atmometers  at  different  stations  in 
the  United  States  is  shown.  "Grove"  station  in  1916  is  fairly  com- 
parable with  "Cultivated"  station  in  1917,  as  is  the  case  with  the  two 
"Desert"  stations.  Further  evidence  of  this  correlation  is  afforded 
by  the  mean  maximum  temperatures  obtaining  during  the  critical 
period  in  the  development  of  the  young  fruit  (table  2).  During  this 
period  in  1917  (June  and  July)  the  mean  maximum  temperatures 
were  95?6  F  and  104?4  F  respectively,  while  those  for  the  critical 
period  in  1916  (May  and  June)  were  only  81?1  F  and  93?0  F. 

TABLE  5 

Comparative  Loss  from  Cylindrical  White  Porous  Cup  Atmometers  at 
Different  Stations  in  the  United  States  for  the  Month  of  June 

Average  daily  loss  for 
Station  24  hours  in  ce. 

Miami,  Fla.*  15.9 

Urbana,  Illinois*  16.1 

' '  Alfalfa  ' '  Station,  East  Bakersfield,  1917....  18.5 

Whittier,  Calif.,  1912  22.8 

Berkeley,  Calif.,  1917  23.1 

West  Raleigh,  North  Carolina* 28.0 

Gainesville,  Florida* 28.7 

"Tree"  Station,  Edison,  1916  32.9 

San  Diego,  Calif.*  33.0 

Cameron,  Louisiana* 33.4 

' '  Tree ' '  Station,  East  Bakersfield,  1917 35.8 

Riverside,  Calif.,  1912  43.4 

Dickinson,  North  Dakota* 45.0 

"Grove"  Station,  Edison,  1916  48.1 

"Yard"  Station,  Edison,  1916  55.1 

"Desert"  Station,  Edison,  1916  69.1 

Eeno,  Nevada* 69.5 

' '  Cultivated ' '  Station,  East  Bakersfield,  1917  71.7 

Tucson,  Arizona*  73.0 

Dalhart,  Texas*  80.7 

"Desert"  Station,  East  Bakersfield,  1917 94.0 

*  Livingston,  B.  E.,  A  Study  of  the  Relation  between  Summer  Evaporation  Intensity 
and  Centers  of  Plant  Distribution  in  the  United  States,  Plant  World,  vol.  14  (1911), 
pp.  205-22. 

This  correlation  is  again  reflected  in  the  comparative  yields  in 
general  throughout  the  state  in  the  sea.sons  of  1916  and  1917.  The 
latter  season  has  been  noted  for  its  long  continued,  high  temperatures 


1919]      Coit-Bodgson :  Abnormal  Shedding  of  Washington  Navel  Orange  309 

and  low  humidity,  while  the  former  was  as  equally  marked  by  its  rela- 
tively low  temperatures  and  equableness.  The  crop  in  1917  over  the 
entire  state  is  not  estimated  to  be  more  than  40  to  50  per  cent  of  that 
in  1916. 

All  the  more  recent  fundamental  work  in  [ilaiit  physiology  has 
indicated  that  for  plants  growing  in  the  open  the  water  relation  is 
the  limiting  factor.  It  is  at  once  obvious  that  under  the  conditions 
obtaining  in  the  arid  southwest  it  is  the  water  relation  which  is  most 
likely  to  be  strained.  This  is  particularly  to  be  considered  in  connec- 
tion with  the  previously  mentioned  fact  that  the  genus  Citrus  is 
lujdoubtedl}'  of  tropical  origin  and  therefore  not  well  adapted  by 
nature  to  withstand  the  tremendous  water  loss  incident  to  the  severe 
climatic  complex  obtaining  under  arid  conditions. 

Evidence  that  abnormal  water  relations  due  to  the  influence  of  the 
environmental  complex  may  furnisli  the  stinuilus  to  abscission  is  not 
lacking.  In  regard  to  the  cotton  phnit  Balls-"*  says:  "It  is  certain 
that  the  main  factor,  if  not  the  only  one,  is  the  water-content  of  the 
plant."  Lloyd,*"  also  working  with  cotton,  concludes  that  "the  water 
deficit  is  the  cause  of  rise  of  temperature  in  the  tissues,  and  this 
constitutes  the  stimulus  which  directly  leads  to  abscission."  llDward^' 
has  noted  the  fact  that  abnormal  water  conditions  in  the  soil  are 
immediately  shown  in  the  indigo  plant.  Indigofcra  arrecta,  by  leaf-fall 
or  by  the  shedding  of  flowers  without  setting  .seed.  His  interpretation 
of  these  results  will  be  referred  to  later.  The  junior  autliDi-  has  already 
presented  data  to  show  that  at  Edison  an  al)normal  water  relation 
does  exist  in  orange  leaves  and  young  fruits  during  the  critical 
period.*-  He  has  shown  that  a  daily  water  deficit  of  25  to  30  per 
cent  occurs  in  the  j-oung  fruits,  wliidi  deficit  is  made  up  at  night. 
These  deficits  are  at  their  maxima  tluring  the  afternoon,  at  which 
period  the  atmospheric  pull  on  the  plant  for  water  is  at  its  maximum. 
A  contributing  factor  to  these  water  deficits  lies  in  the  fact  that  under 
stress  of  the  tremendous  atmospheric  pull  for  water  the  h'avcs  actually 
apjiropriate  water  from  the  young  fruits.  This  strain  on  the  plant  is 
not  localized  but  extends  throughout  the  tree.  Tensions  developed  by 
exterior  foliage  are  transmitted  quickly  to  interior  fruits  and' even  to 
distant  roots  as  was  shown  by  several  exin-riiiiiiils:  for  llic  sake  of 
brevity  only  one  will  be  described. 


3»  Loc.  cit.,  p.  69. 

«)  Tlie  Abscis.sion  of  Flower-buds  and  Fruits  in  Gossypium,  and  its  Relation 
to  Environnioiitiil  Clian^'ps,  Trans.  Roy.  Soc.  Canada,  aer.  3,  vol.  10  (1916),  p.  CI. 
<i  Soil  Aeration  in  Agriculture,  Agr.  Res.  Inst.  Pusa,  Bull.  61,  1916. 
*-  Loc.  cit. 


310 


University  of  California  Publications  in  Agricultural  Sciences      [Vol.  3 


On  May  4,  1916,  a  large  number  of  ;)pi)areiitly  healthy  terminal 
fruits  about  one-half  inch  in  diameter  were  selected  on  six  trees  at 
Edison.  Lot  A  was  left  as  a  cheek,  lot  B  was  treated  by  clipping  off 
with  scissors  one-third  of  the  leaves  of  the  current  season's  growth 
liehind  the  fruit.  Lot  C  suffered  excision  of  two-thirds  of  the  leaf  area, 
and  lot  D  had  all  of  the  leaves  removed,  leaving  the  fruit  terminating  a 
bare  stem  about  six  inches  long.  Unfortunately,  a  few  of  the  limbs 
in  these  trees  were  removed  by  tree  primers.  On  November  16  the 
remaining  labels  were  located  and  a  record  made  of  the  number  of 
fruits  which  persisted  to  maturity. 

TABLE  6 
Effect  of  Reduction  of  Adjacent  Leap  Area  on  Abscission 


A.  Checlv,  not  treated  

B.  One-third  leaf  area  removed.... 

C.  Two-thirds  leaf  area  removed 

D.  All  leaves  removed 


Nximber 
labeled 

Number  labels 
found 

Number  of 
fruit  set 

200 

16.5 

29 

200 

163 

19 

200 

176 

37 

200 

167 

21 

Thus  we  see  that  the  reduction  of  adjacent  leaf  area  had  no  effect 
on  the  drop  and  inasmuch  as  eosin  solution  was  drawn  through  oranges 
in  situ  to  the  stem  in  lot  D  very  nearly  as  quickly  as  in  lot  A,  it  is 
evident  that  the  tensions  referred  to  have  to  do  with  the  tracheal 
system  as  a  whole,  as  is  to  be  expected,  and  do  not  tend  to  be  localized 
ill  any  particular  part  of  the  tree. 


TABLE  7 

Presence  of  Lithium  Nitrate  in  Leaves  Behind  Fruits  Injected  with 

Dry  Crystals  as  Shown  by  the  Spectroscope 

Experiment  began  at  12  m. 


Time 

1 2  -  30  P  M 

First 
leaf 

Posi- 
tive 

Posi- 
tive 

Posi- 
tive 

Posi- 
tive 

Posi- 
tive 

Very 
slitriit 
trace 

Nega- 
tive 

Second 
leaf 

Posi- 
tive 

Third 
leaf 

Posi- 
tive 

Fourth 
leaf 

Nega- 
tive 

Fifth 
leaf 

Nega- 
tive 

Posi- 
tive 

Sixth 
leaf 

Seventh 
leaf 

Nega- 
tive 

Twelfth 
leaf 

30  minutes 

1-00  P.M. 

Nega- 
tive 

1  hour 
2*00  P  M 

Posi- 
tive 

Posi- 
tive 

Nega- 

2 hours 

tive 

3  hours 

Posi- 
tive 

Posi- 
tive 

4  hours 

12:00  m. 
24  hours 

Check 

Verv 

slight 

trace 

Nega- 
tive 

Nega- 
tive 

Trace 

(?) 

1919]      Coii-Hodgson:  Abnormal  Shedding  of  Washington  Narel  Orange  311 

The  withdrawal  of  water  from  the  fruits  by  the  leaves  has  been 
further  substantiated  by  the  use  of  dry  crystals  of  lithium  nitrate 
injected  into  the  navel  end  of  the  youn^r  fruits  and  t(>stin.fr  foi-  lithium 
in  the  leaves  proximal  to  the  fruits  at  different  periods  following  in- 
jection by  means  of  the  spectroscope.  These  results  are  summarized 
in  table  7,  where  it  can  be  seen  that  withijj  a  half  hour,  in  spite  of  the 
fact  that  the  lithium  nitrate  was  injected  dry  into  the  fruit  and  had 
to  go  into  solution  in  the  freed  cell  sap.  its  presence  was  shown  in  the 
leaves  behind  the  fruits. 

"Water  relations  of  this  same  general  sort  have  been  established  by 
a  number  of  other  investigators  in  plants  where  such  deficits  do  not 
constitute  a  stimulus  to  abscission.  Under  this  category  are  to  be 
classed  Renner's"  "siitigungsdefizit"  and  the  phenomenon  of  "in- 
cipient drying"  described  b.y  Livingston  and  Brown"  and  established 
in  other  plants  by  Lloyd"  and  Edith  B.  Shreve." 

To  determine  actually  the  ultimate  connection  between  abnormal 
water  relations  of  the  type  noted  and  the  abscission  of  young  fruits  has 
constituted  a  most  difficult  problem,  and  the  evidence  indicating  such 
a  connection  has  been  obtained  from  several  different  lines  of  attack. 
Although  not  as  conclusive  as  could  be  desired,  still  we  believe  that  it 
is  sufficient  to  indicate  in  general  the  relation  between  the  two.  It  is 
hoped  that  additional  evidence  can  be  obtained  during  the  next  season, 
which  evidence  we  were  unable  to  get  during  our  investigation  through 
lack  of  .sufficient  equipment  and  apparatus. 

As  was  mentioned  in  the  description  of  the  East  Bakersfield  station, 
this  orchard  is  planted  to  alfalfa,  protected  by  an  efficient  windbreak, 
and  heavily  irrigated.  The  noteworthy  fact,  however,  is  that  this 
orchard  habitually  hears  crops  in  every  way  comparable  to  orchards  of 
the  same  age  and  general  treatment  located  near  the  coast.  Although 
situated  only  three  and  one-half  miles  from  the  Edison  station  and 
having  the  same  exposure,  the  trees  being  one  year  younger,  and  all 
conditions  similar  in  every  way  with  the  exceptions  noted,  this  orchard 


<3  Experimentelle  Beitrage  ziir  Konntnis  dor  Wagscrbeweguiig,  Flora,  vol.  103 
(1911),  pp.  171-247. 

**  Kol.Ttion  of  the  daily  march  of  transpiration  to  variations  in  tho  water 
content  of  foliatjo  leaves,  Bot.  Gaz.,  vol.  53  (1912),  pp.  309-30. 

<!>  The  Relation  of  Transpiration  and  Stoniatal  Movement  to  the  Water  Con- 
tent of  the  Leaves  of  Foiiquierin  splcndcim,  Plant  World,  vol.  15  (1912),  pp. 
1-14;  Leaf  Water  and  Stomatal  Movement  in  Goss/ipium  and  a  Method  of  Direct 
Visual  Observation  of  Stomata  in  situ,  Bull.  Torr.  Bot.  Club,  vol.  40-  (I9I3),  pp. 
1-26. 

<«  The  daily  march  of  transpiration  in  a  desert  jiorennial,  Carnegie  Inst. 
Washington,  i'ubl.  194,  1914. 


312 


University  of  California  Publications  in  Agricultural  Sciences      [Vol.  3 


bears  heavj'  crops  (pi.  26),  and  luis  been  profitable  ever  since  it  came 
into  bearing  three  to  four  .years  ago. 

The  conclusion  cannot  but  be  forced  that  in  the  exceptions  noted 
lies  the  secret  of  the  heavy  set  of  fruits.  In  order  to  obtain  some  idea 
of  the  climatic  conditions  obtaining  within  this  orchard  as  compared 
with  those  under  Edison  conditions  we  had  recourse  to  what  metero- 

no  t— 


80 


70 


40 


30 


20 


I    I    I    I    I    I   I 


.MAY 


J I  XL 


Fig.  2.     Comparison  of  daily  atmometer  water  loss  at  four  different  stations 
at  Kdison  in  1916.    Ordinates,  water  loss  in  cc;  abscissae,  days  of  the  month. 


logical  instruments  were  available  to  us.  While  much  more  significant 
results  could  have  been  obtained  had  we  posses,sed  more  equipment,  we 
feel  that  our  data,  while  possibly  not  accurately  quantitative,  at  least 
are  qualitative  enough  to  justify  our  conclusions.  Air  temperature 
and  humidity  readings  were  taken  by  means  of  a  Freiz  thermo-hygro- 
graph.  "We  were  particularly  interested,  however,  in  the  integration 
of  all  the  climatic  factors  in  their  eflFect  upon  the  plant  and  for  this 
purpose  selected  the  Livingston  white  cylindrical  porous  cup  atmo- 


1919]       Coit-Hodgson :  Abnormal  Sheddiiiff  of  Washington  Navel  Orange  313 

meter*"  (pi.  41).  We  are  cognizant  of  criticisms  (if  this  instrument 
by  Briggs  and  Shantz.**  but  believe  that  for  our  purpose  it  is  suffic- 
iently accurate.  Due  to  a  lack  of  a  sufficient  number  of  these  instru- 
ments we  were  unable  to  run  a  series  simultaneously  at  Edison  and 
at  East  Bakersfield  Init  we  did  oix'rate  tliem  under  as  nearly  similar 
conditions  at  the  lattei-  place  in  ]f)17  as  at  the  former  in  1!)16.    Know- 


^~ 

70 

: 

DESERT 

GO 

— 

— 

YARD 

SO 

GROVE 

40 

— 

— 

TREE 

30 

20 

— 

10 

— 

Fig.  3.     Cumparison  of   the  average   daily  atiiiometer  water  loss   from   the 
stations  referred  to  in  figure  2. 


ing  something  of  the  relative  harshness  (if  llic  two  seasons,  lioth  as 
reflected  in  the  amount  of  dropping  and  in  the  data  taken  I)y  the  U.  S. 
Weather  Bureau  observer  at  Bakersfield,  we  are  able  to  approximate 
fairly  well  the  climatic  conditions  at  Edison  in  1917  for  comparative 
purpases.  The  water  loss  from  our  different  stations  at  the  two  locali- 
ties is  well  shown  in  figures  2,  3,  4.  and  5  and  in  table  5. 

At  Edison  our  atmometer  stations  were  selected  as  follows:  "Tree" 
station  was  located  underneath  an  orange  tree  near  the  center  of  tiie 
orchard,  about  one-half  mile  to  leeward  of  the  edge  of  the  orchard 


<' The  Relation  of  Desert  Plants  to  Soil  Moisture  ami  to  Evaporation,  Car- 
negie Inst.  Washington,  I'ulil.  .^0,  190(). 

<8  Comjiarison  of  the  Hourly  Kvaiioration  Kate  of  Atnioineters  and  Free 
Water  .Surfaces  with  the  Transpiration  Kate  of  Medicago  soti\-(i.  Jour.  Agr.  Kes., 
vol.  0  (ISUT),  pp.  277-90. 


314 


University  of  California  Publications  in  AgricuJtural  Sciences      [Vol.  3 


which  bordered  the  desert.  "Grove"  station  wa.s  .situated  in  the  open 
orchard  midway  between  the  tree  just  mentioned  and  its  neighbor. 
"Desert"  station  wa.s  located  on  tlie  open,  bare  desert  about  one-half 


100 


90 


70 


0(1 


30 


20 


TREE 


I        I        I        I         I        I        I        I        I 


10 


JILY 


Fig.  4.  Daily  evaporation  from  atmometers  at  four  different  stations  at 
East  Bakersfield  in  ]017.  Ordinates,  water  loss  in  cc;  abscissae,  days  of  the 
month. 


jnile  to  windward  of  the  edge  of  the  orchard  and  many  miles  to  leeward 
of  anj'^  irrigated  land  (pi.  41)).  The  data  accumulated  for  nineteen 
days  are  shown  in  figures  2  and  3  and  table  5. 


1919]      Coit-Bodgson :  Abnormal  Shedding  of  Washington  Navel  Orange  315 

At  PJast  Bakersfield  our  atmometers  were  set  \ip  at  the  following 
stations:  "Tree"  station  was  similar  to  "Tree"  station  at  Edison 
except  that  the  tree  where  it  was  located  was  in  the  orchard  planted 
to  alfalfa.  "Alfalfa"  station  was  located  similarly  to  "Grove" 
station  at  Edison  but  of  course  was  surrounded  on  all  sides  by  alfalfa, 


' 

' 

— 

DESERT 

90 

80 

— 

CULTIVATED 

70 

60 

— 

.w 

— 

40 

TREE 

30 

— 

20 

ALFALFA 

I'l 

— 

' 

— 

Fi}:.  5.     Average  daily  water  loss  from  atmometers  at  the  stations  referred 
to  in  figure  4. 


which  averaged  some  twelve  to  eighteen  inches  high.  "Cultivated" 
station  was  located  in  every  respect  similarly  to  "Grove"  .station  at 
Edison  and  the  two  "Desert"  stations  were  similarly  situated.  The 
data  accumulated  for  fourteen  days  are  shown  in  figures  4  and  5  and 
table  5. 


316  University  of  California  Publications  in  Agricultural  Sciences      [Vol.3 

It  is  at  once  obvious,  looking  at  the  stations,  whicii  arc  in  every 
way  comparable,  that  the  critical  period  in  1917  was  considerably  more 
severe  than  in  1916  (fig.  6),  which  difference  has  been  pointed  out  with 
respect  to  the  yield  of  the  Edison  orchard.  It  is  also  equally  evident 
that  the  water  los.s  from  the  soil  and  plants  has  a  most  profound  effect 
in  ameliorating  the  atmospheric  evaporating  power  and  that  this  effect 
is  cumulative  with  the  direction  ofi  the  prevailing  winds.  Thus  at 
Edison  the  "Desert"  atmometer  lost  an  average  of  69.1  ec.  to  48.1  ee. 
lost  by  the  "Grove"  station  and  at  East  Bakersfield  the  .same  stations 
lost  water  in  the  ratio  of  94.0  ec.  to  71.7  cc.  At  Edison  the  orchard 
environment  during  1916  was  sufficient  to  cut  down  the  asperity 
of  the  climate  about  45  per  cent,  while  at  the  Kellogg  place  in 
1917  it  was  sufficient  to  reduce  it  31  per  cent.  The  atmometer 
inside  the  tree  lost  only  two-thirds  of  that  lost  by  the  instrument  at 
"Grove"  station  or  only  45  per  cent  of  that  at  the  "Desert"  station. 
Thus  we  can  see  the  marked  effect  of  an  orchard  in  modifying  its 
own  environmental  complex.  It  is  undoubtedy  this  influence  which 
the  orchard  manifests  per  se  which  explains  to  some  degree  why  it  is 
that  as  orchards  planted  in  exposed  districts  grow  older,  the  percentage 
of  yield  increases  more  than  the  increase  in  size  of  tree.  The  fact  that 
inside  fruit  is  subjected  to  an  entirely  different  climate  than  exposed 
fruit  serves  to  explain  why  it  is  notably  of  better  texture  and  grade 
and  why  it  possesses  so  few  large  and  protuberant  navels.  We  have 
observed  that  Navel  oranges  grown  in  the  University  of  California 
greenhouses  are  of  markedly  superior  texture  and  navel  conformation 
to  those  produced  outside,  where  conditions  are  not  so  mild  or  uniform. 
Again,  it  is  this  cumulative  modification  of  the  climatic  complex  fol- 
lowing the  direction  of  the  prevailing  wind  which  explains  the  fact 
that  a  notably  heavier  set  of  fruit  occurs  on  the  .south  and  east  side 
of  the  trees.  This  condition  has  been  frequentlj'  mentioned  and  was 
quite  marked  at  Edison  in  1017. 

But  the  most  striking  modifications  in  climatic  conditions  are  to  be 
seen  with  reference  to  the  situation  at  East  Bakersfield.  Although  the 
Desert  station  atmometer  lost  an  average  of  94.0  cc.  the  Alfalfa  station 
instrument  lost  only  18.5  cc.  or  only  20  per  cent  as  much.  Reference 
to  table  5  serves  to  show  that  here  is  a  climatic  change  within  a 
half  mile  in  the  San  Joaquin  desert  of  the  same  magnitude  as  that 
between  Miami,  Florida,  and  Tucson,  Arizona.  The  effect  is,  of  course, 
largely  due  to  the  fact  that  the  alfalfa  transpires  at  a  tremendous  rate 
and  the  atmometer  cup  at  that  station  was  continuously  bathed  in  an 


1919]       Coit-Bodgson :  Abnormal  Shedding  of  Jrashington  Navel  Orange  317 


> 
31 


> 
■< 


•4       o> 


c 
r* 
■< 


318  University  of  California  Publications  in  Agricultural  Sciences      [Vol.  3 

almost  saturated  atmosphere.  The  windbreak  served  to  prevent  the 
blanket  of  moist  air  from  being  rapidly  dissipated.  The  loss  from 
Tree  staticm  is  seen  to  be  35.8  cc,  or  only  30  per  cent  of  that  lost  by 
the  Desert  instrument.  Although  the  effect  of  the  alfalfa  cannot  be 
exerted  at  any  very  considerable  height  above  the  ground,  still  it  is 
certain  that  the  orange  trees  (with  the  young  developing  fruits')  sur- 
rounded by  this  transpiring  alfalfa  are  literally  bathed  in  a  damp 
atmosphere;  at  any  rate  so  far  as  the  tree  is  concerned  it  is  subjected 
to  a  very  different  climate  from  that  which  obtains  on  the  desert.  The 
influence  of  the  alfalfa  in  modifj'ing  the  atmospheric  humidity  can 
clearly  be  seen  when  the  crop  of  oranges  is  picked,  for  under  these 
conditions  most  of  the  fruit  is  borne  near  the  ground  and  less  in  the 
tops  of  the  trees.  At  Tree  station.  East  Bakersfield,  thermo-hygro- 
graph  readings  were  taken  for  a  period  of  twenty  days.  A  study  of 
the  record  for  the  period  of  the  investigation  shows  some  interesting 
results.  At  no  time  did  the  temperature  rise  above  107°  F  although 
in  the  laboratory,  a  quarter  of  a  mile  away,  temperatures  of  110°  to 
112°  F  were  registered  several  times.  The  most  significant  feature, 
however,  is  the  relative  humidity  curve.  The  lowest  humidity  reached 
was  25  per  cent,  wliich  occurred  at  the  time  that  the  107°  F  tempera- 
tures were  recorded,  July  9  and  21.  The  average  relative  humidity 
during  the  day  was  between  40  and  50  per  cent.  In  1916  at  Edison 
we  recorded  humidities  as  low  as  10  per  cent  and  the  average  relative 
humidity  was  between  25  and  35  per  cent.  It  is  unfortunate  that  we 
were  not  able  to  obtain  simultaneous  temperature  and  humidity  read- 
ings at  the  Desert  station  in  1917,  but  in  view  of  the  fact  that  the 
1917  season  has  been  shown  to  be  much  more  severe  than  the  1916 
season  there  is  little  doiibt  that  in  1917  the  relative  humidity  was 
somewhat  lower  and  the  temperature  somewhat  higher  than  in  the 
former  season. 

"We  recognize  clearly  that  in  agricultural  enterprises  it  is  unsafe 
to  rely  upon  climatic  averages.  It  is  well  known  that  with  some  crops 
suece.ss  or  failure  depends  largely  upon  the  extremes  of  climatic  con- 
ditions experienced  during  a  certain  critical  period  in  their  growth. 
However,  it  should  be  borne  in  mind  that  conditions  which  tend  to 
ameliorate  the  environmental  complex  not  only  raise  the  general 
average  favorably,  but  also  have  a  distinct  modifying  effect  upon 
extremes  in  weatlier  conditions  which  may  occur.  Indeed,  it  seems 
probable  that  this  is  the  most  important  effect  of  the  alfalfa  and 
windbreaks  in  the  Kellogg  orchard.     It  is  not  so  much  the  liigher 


1919]       Coit-Eodgson:  Abnormal  Shedding  of  Washington  Navd  Orange  319 

average  humidity  as  it  is  the  greater  freedom  from  extreme  variation 
in  climatic  conditions  which  serves  to  enable  the  young  fruits  to 
survive. 

As  referred  to  above,  the  junior  author*"  has  shown  in  another  place 
that  a  marked  water  deficit  occurs  both  in  the  young  fruits  and  the 
leaves  under  the  climatic  conditions  obtaining  at  Edison  and  has  sug- 
gested that  these  abnormal  water  relaticTus  furnish  the  stimulus  to 
abscission.  If  this  be  so,  then  when  there  is  little  or  no  dropping  of 
the  fruits  and  consequent)}'  a  good  crop,  such  abnormal  water  relations 
should  not  be  found.  An  effort  was  made  at  the  East  Bakersfield 
station  in  1917  to  establish  such  abnormal  water  relations,  but  it  was 
found  impossible  to  do  so  (table  8).    Instead  of  there  being  a  regular 

TABLE  8 
Average  Moisture  Content  at  Different  Times  op  Day 

Average  water  content  in 
per  cent,   calculated  on 
basis  of  dry  weight 
Kind  of  material  1916  1917 

Normal  fruits  one-third  to  three-fourths  inch  in  ilianieter 

gathered  before  noon  _ 260.2  285.3 

Same,  but  gathered  after  noon 247.0  283.9 

Leaves  of  current  season's  growth,  gathered  before  noon  164.9  174.9 

Same,  but  gathered  after  noon 157.2  182.6 

decrease  in  water  content  of  similar  leaves  and  fruits  during  the  day, 
which  is  made  up  during  the  night,  no  such  relation  was  found.  At 
East  Bakersfield  the  leaves  and  fruits,  in  the  first  place,  averaged 
somewhat  higher  in  moisture  content  than  those  taken  at  Edison. 
Secondly,  although  as  nearly  similar  in  every  respect  as  possible, 
duplicate  series  showed  an  absolute  lack  of  uniformity,  the  variation 
sometim(!s  being  as  much  as  30  to  40  per  cent.  Finallj',  no  average 
decrease  in  water  content  either  of  the  fruits  or  leaves  was  found 
to  occur  during  the  day.  It  should  be  mentioned  that  irrigation  at 
the  Kellogg  place  is  not  uniform,  relatively  small  tracts  being  irri- 
gated at  one  time  and  these  thoroughly  soaked.  As  it  was  found  in- 
convenient to  take  all  the  leav&s  and  fruits  from  the  same  trees  it  is 
possible  that  .some  of  the  variation  in  moisture  content  noted  may  be 
attributed  to  variations  in  soil  moi.sture.  Ilowevei",  under  the  marked 
modification  of  climatic  conditions  which  has  been  shown  to  occur  as 
a  result  of  the  management  of  the  orchard,  it  is  believed  that  such 
abnormal  water  relations  do  not  occur,  at  least  to  anytliing  liUe  the 
extent  to  which  they  do  under  the  mimodified  climatic  conditions. 

<»  Loc.  cit. 


320  University  of  Ccilifoniia  Publications  in  Agricultural  Sciences      [Vol.  3 

As  to  the  ultimate  stimulus  beyond  alinonnal  water  relations  we 
can  do  little  but  speculate.  Lloyd'"  has  expressed  the  idea  that  increase 
in  temperature  following  water  deficits  may  be  the  ultimate  stiniidus 
to  abscission.  It  ha.s  long  been  known  that  plant  parts,  when  for  any 
reason  deprived  of  a  normal  supply  of  water,  suifer  an  increase  in 
internal  temperature.  In  an  effort  to  furnish  additional  evidence  as 
to  the  presence  of  abnormal  water  relations,  as  well  as  to  obtain  some 
idea  of  the  temperature  changes  incident  to  such  water  deficits,  we 
took  some  temperatures  of  fruits  destined  to  fall,  fruits  suffering 
from  a  water  deficit  by  reason  of  the  fact  that  the  tree  was  permitted 
to  suffer  for  lack  of  irrigation,  and  temperatures  of  normal  fruits. 
These  are  found  summarized  in  tables  9  and  10.     It  will  lie  seen  that 


TABLE 

9 

Interior  Temperatures  of 

Fruits, 

Fahrenheit 

Hour 

Fruit  destined                  Xorma! 
to  drop                   liealtiiy  fruit 

Air 

9 

91.8 

91.4 

91.5 

9:20 

94.1 

91.5 

93.2 

10 

96.3 

93.0 

95.9 

11 

100.4 

96.9 

97.5 

12 

102.2 

98.0 

100.4 

1 

106.5 

100.9 

105.8 

2 

110.5 

104.9 

no.i 

3 

109.9 

107.2 

109.0 

4 

111.9 

110.3 

110.8 

5 

111.2 

110.3 

107.2 

5:30 

107.6 

107.6 

106.2 

Average 

103.8 

TABLE 

101.0 
10 

ini.fi 

Imterior  Temperatures  of  Fruits, 

Fahrex 

heit 

Hour 

Fruit  suiferinfr 
from  drougllt 

Normal 
lealtliy  fruit 

Air 

8 

87.3 

86.9 

90.1 

9 

91.4 

90.5 

92.1 

10 

95.5 

95.0 

97.5 

n 

98.6 

97.2 

100.2 

12 

102.9 

100.4 

103.6 

1 

104.2 

103.5 

105.2 

2 

J  04.9 

104.0 

107.6 

.■? 

106.2 

105.8 

107.6 

4 

104.0 

104.0 

104.0 

r> 

101.3 

101.3 

101.6 

6 

98.6 

98.6 

98.2 

/ 

93.2 

93.2 

94.1 

Average 

99.0 

98.2 

100.1 

so  Loe. 

cit. 

1910]       Coit-Hodgson :  Abnormal  Slicddintj  of  Washington  Navel  Orange  321 

the  normal  fruits  average  soniewliat  lower  in  temperature  than  the 
air,  and  in  turn  those  destined  to  drop  are  somewhat  higher  in  tem- 
perature than  the  air.  Fruits  permitted  to  suffer  for  lack  of  water 
show  a  temperature  approximately  that  of  the  air  surrounding  them. 
It  may  he  that  increase  in  temperature  due  to  water  deficits  is  the 
ultimate  stimulus  to  ahseission,  still  it  should  be  pointed  out  that  the 
increases  in  temperature  as  recorded  by  us  are  of  a  much  smaller 
magnitude  than  the  daily  range  in  temperature  changes.  We  are 
fully  aware,  of  course,  that  strictly  accurate  temperatures  of  plant 
tissues  can  only  be  obtained  by  thermo-electric  means,  the  mercury 
thermometer  being  too  sul).ject  to  tiuctuation  and  variation  for  very 
delicate  work. 

Factors  Operative  :n  Causing  Water  Relation  Strains 

It  is  of  course  obvious  that,  given  a  plant  transpiring  a  certain 
amount  of  water  vapor  daily,  unless  there  be  a  sufficient  water  supply 
in  the  soil  witliin  reach  of  the  absoi-bing  roots  to  make  up  for  that  lost 
by  the  plant  and  in  addition  supply  enough  for  its  metabolic  processes, 
water  deficits  of  the  kind  mentioned  nuist  eventually  occur.  That 
under  these  conditions  such  do  occur  and  that  they  are  followed  by  an 
abnormally  severe  shedding  of  the  young  fruits  when  in  the  critical 
period,  is  the  ob.servation  of  the  authors  and  the  experience  of  many 
growers.  In  the  season  of  191(5  the  junior  author  had  under  observa- 
tion a  ten-acre  block  of  orange  trees  in  the  Orovill(>  district  which 
had  been  top  worked  to  the  Washington  Navel  variety  five  years 
previously.  They  bloomed  very  heavily  and  set  an  excellent  crop. 
Through  an  accident  to  the  irrigation  system  preventing  a  sufficient 
supply  of  water  these  trees  were  allowed  to  suffer  for  lack  of  water 
at  the  time  when  the  young  fruits  were  about  one  centimeter  in 
diameter.  At  the  time  of  irrigation  several  days  later  the  fruits  had 
not  fallen  and  it  was  hoped  that  the  crop  could  be  saved.  Within  a 
week  practically  every  fr-uit  was  shed,  altlmugh  the  trees  looked  well 
and  had  entirely  recovered  from  the  drought. 

Observations,  confirmatory  in  every  resi)ect  to  those  given  above, 
were  made  on  a  row  of  trees  at  the  Kellogg  place  in  1917.  These  trees 
were  permitted  to  suffer  for  lack-  of  irrigation.  Although  the  only 
trees  in  the  row  which  at  the  time  bore  fruits  in  the  critical  stage  were 
of  the  Valencia  variety,  which  variety  is  nuu-h  less  subject  to  shedding 
than  the  Washington  Navel,  still  within  a  week  after  the  application 
of  the  water  many  of  the  young  fruits  had  fallen.     The  desirability  of 

21 0063 


322  University  of  California  Publications  in  Agricultural  Sciences      [Vol.  3 

a  proper  moisture  supply  in  the  soil  at  the  blooming  and  setting  period 
is  reflected  in  the  practice  of  many  growers  who  irrigate  their  orchards 
heavily  at  such  times  as  well  as  during  the  periods  of  hot,  dry  north 
winds. 

In  this  connection  it  should  be  noted  that  Fowler  and  Lipman'*^ 
have  recently  shown  that  under  conditions  of  a  soil  moisture  supply 
somewhat  below  the  optimum  the  visible  effects  upon  the  citrus  tree 
are  a  great  deal  less  than  under  conditions  of  the  same  percentage 
above  the  optimum  moisture  content.  In  other  words,  these  authors 
have  shown  that  the  citrus  tree  does  not  exhibit  the  effects  of  a  deficient 
soil  moisture  supply  to  the  same  extent  that  it  does  an  excess  of 
moisture  in  the  soil.  It  may  well  be,  therefore,  that  many  of  our 
citrus  orchards  are  underirrigated  and  the  irregular  water  relations 
above  discussed  accentuated  by  reason  of  this  fact.  The  authors  feel 
that  many  of  the  orchards  studied  in  this  investigation  would  probably 
do  better  with  heavier  irrigation.  Manifestly  it  would  be  useless  to 
attempt  methods  of  modifying  the  climatic  complex  with  the  end  in 
view  of  cutting  down  daily  water  deficits,  if  the  soil  moisture  supply 
is  deficient.  Therefore,  the  grower  should  first  make  certain  that 
sufficient  soil  moisture  is  available. 

It  has  long  been  known  that  the  presence  of  sufficient  moisture  in 
the  soil  is  not  conclusive  evidence  that  the  plant  is  enjoying  optimum 
moisture  conditions.  Plants  inhabiting  salt  marsh  regions  po-ssess  their 
xerophytie  adaptations  by  reason  of  the  fact  that  although  growing 
with  their  roots  in  water  or  mud  they  are  unable  to  obtain  water  in 
anj'  large  amounts  and  are  forced  to  economy  in  the  use  of  it.  This 
inability  to  absorb  water  has  been  traced  to  the  ratio  between  the 
osmotic  concentrations  of  the  soil  solution  and  the  cell  sap  of  the  roots, 
and  such  a  condition  is  called  "physiological  drought."  Physiological 
drought  may  be  induced  by  the  inhibition  of  absorption  through  the 
action  of  factors  other  than  the  osmotic  concentration  of  the  solutions 
involved. 

Among  the  most  important  factors  conditioning  absorption  is  that 
of  aeration.  It  has  long  been  known  that  when  gro\vn  in  water  cul- 
tures many  plants  make  very  unsatisfactory  growth.  Hall,  Brenchley, 
and  Underwood''-  have  recently  shown  that  this  un.satisfactory  growth 
is  due  to  lack  of  aeration  and  can  be  remedied  by  passing  a  stream  of 


SI  Optimum  Moisture  Conditions  for  Young  Lemon  Trees  on  a  Loam  Soil, 
IJniv.  Calif.  Publ.  Agr.  Sci.,  vol.  3  (1917),  pp.  25-36. 

''-  The  Soil  Solution  and  the  Mineral  Constituents  of  the  Soil,  Jour.  Agr.  Sci., 
vol.  6  (1914),  pp.  296-301. 


1919]       Coit-Bodgson :  Abitormal  Shedding  of  Washington  Navel  Orange  323 

air  through  the  solution.  The  economic  applications  of  this  principle 
are  many,  but  are  of  course  particularly  evident  in  regions  where 
through  special  conditions  lack  of  soil  aeration  is  emphasized,  as  is  the 
case  in  certain  parts  of  India.  The  soil  is  naturally  very  heavy  and 
easily  packed  by  the  torrential  rains.  Lack  of  aeration  is  accentuated 
during  certain  portions  of  the  growing  season  by  the  occurrence  of 
monsoons  and  tropical  rainstorms  of  great  severity.  Howard"^  has 
shown  most  conclusively  that  under  these  conditions  the  production 
of  the  gram  or  chick-pea,  Cicer  arietinum,  grown  to  the  extent  of  over 
eighteen  million  acres,  is  absolutely  conditioned  by  the  soil  aeration. 
If  the  soil  is  permitted  to  become  packed  by  summer  rains  and  the 
air  supply  cut  off,  tlic  jilants  wilt  down  with  water  actually  stand- 
ing on  the  surface  of  the  soil.  Absorption  is  cut  dow-n  to  practically 
nothing,  while  transpiration  is  not  reduced  in  the  same  ratio,  resulting 
in  ultimate  wilting.  AVhile  not  extensive,  all  the  experimental  data 
available  on  the  production  of  this  crop  in  California  show  this  same 
intolerance  of  lack  of  soil  air.  Howard  has  shown  this  same  condition 
affecting  fruit  trees  and  other  crops,  among  which  is  the  indigo  plant. 
Pree^*  has  shown  that  with  Coleus  hlumei  "even  a  very  small  decrease 
of  oxygen  below  that  normal  to  the  atmosphere  is  injurious  to  the 
plant.  Thus  a  plant,  the  roots  of  which  were  supplied  with  gas  con- 
sisting of  75  per  cent  air  and  25  per  cent  nitrogen,  was  injured  within 
three  days  and  killed  within  45  days.  With  lower  oxygen  content  in 
the  soil  atmosphere  injury  and  death  ai-e  still  more  prompt."  In 
many  cases  the  lack  of  aeration  is  first  evidenced  by  the  shedding  of 
the  leaves  and  flowers.  Soils  of  arid  regions  in  general  are  well 
aerated,  and  especially  soils  of  open  structure  such  as  sands  and 
sandy  loams.  Therefore  it  is  not  likely  that  lack  of  .soil  aeration  is 
the  factor  conditioning  absorption  of  water  by  citrus  trees.  However, 
this  prol)li'in  is  now  under  investigation  and  will  be  reported  on  later. 
I'ihIci-  most  coiiditinns  of  lack'  of  aeration  not  only  is  oxygen 
deficient  but  carbon  dioxide  is  present  in  excess.  The  experimental 
data  available  seem  to  indicate  that  while  in  general  lack  of  oxygen 
and  excess  of  carbon  dioxidi;  in  the  soil  atmosphere  are  detrimental, 
there  is  no  set  rule.     Cannon,'''  and  Livingston  and  Free'"  have  sho\vn 


63  Soil  Aeration  in  Agriculture,  Agr.  Res.  Inst.  Pusa,  Bull.  61,  1916. 

s*  Cannon,  W.  A.,  and  Free,  E.  E.,  The  Ecological  Significance  of  Soil  Aera- 
tion, Science,  n.s.  vol.  45  (1917),  pp.  178-80. 

06  On  the  Relation  between  tlie  Rate  of  Root-Growth  and  the  Oxygen  of  the 
Soil,  Ann.  Rep.  Dir.  Dept.  Hot.  Res.,  Carnegie  Inst.  Washington,  Yearbook  l.") 
(1916),  pp.  74-75. 

'■0  Relation  of  Soil  Aeration  to  Plant-Orowth,  ibid.,  p.  78. 


324  V  III  If  IS  till  of  California  Publicctioiis  in  Agricultural  Sciences      [Vol.3 

that  there  is  considerable  variation  in  tliis  respect,  some  plants,  siieh 
as  Salix  sp.,  growing  and  thriving  in  a  soil  containing  no  oxygen. 
Apparently  the  limiting  concentrations  of  these  gases  must  be  worked 
out  for  each  plant  separately.  As  to  the  specific  effect  of  lack  of 
oxygen  and  excess  of  carbon  dioxide  resulting  in  changes  in  absorption 
rate  little  is  definitely  knowni.  The  first  effect  seems  to  be  a  slowing 
down  of  growth,  which  in  turn  being  ordinarily  accompanied  by  the 
imbibition  (in  the  case  of  the  embryonic  grow-ing  regions  of  the  root) 
of  water  in  considerable  amounts,  reduces  absorption  markedly.  The 
exact  relation  between  growth  and  absorption  is  not  well  undenstood 
at  the  present  time ;  but  it  has  been  shown  by  ilacDougal''  and  c)thers 
of  the  Carnegie  Institution  that  growth  of  embryonic  tissues  is  mainly 
accomplished  by  the  imbibition  of  large  quantities  of  water.  It  can  be 
readily  seen,  therefore,  that  if  conditions  are  unfavorable  for  growth, 
imbibition  and  absorption  must  necessarily  be  reduced. 

Another  factor  which  acts  in  a  very  similar  way  t(i  hick  of  aeration. 
and  one  little  appreciated  up  to  the  present  time,  is  that  of  soil  tem- 
perature. Every  year  adds  more  confirmatory-  evidence  to  prove  that 
the  temperature  relations  of  physiological  proces,ses  follow  certain 
typical  curves,  which  seem  to  be  identical  or  closely  related  for  processes 
of  the  same  fundamental  nature  in  different  organisms.  The  effects 
of  temperature  on  physiological  processes,  both  in  plants  and  animals, 
have  been  investigated  by  many  workers  and  in  general  a  modified 
curve  of  the  Van't  Hoff  type  has  been  obtained  where  the  most  careful 
work  ha.s  been  done.  In  such  curves  several  cardinal  points  can  be 
determined,  namely,  the  minimum  temperature  at  which  the  process 
goes  on,  the  maximum  temperature  beyond  which  the  process  no  longer 
continues,  and  the  oj)timum  temperature  at  which  the  process  is  most 
active.  This  last  term  has  been  superseded  by  what  is  known  as  the 
maxinuim  rate  temperature,  representing  that  temperature  above  which 
the  rate  is  ultimately  decreased  and  below  which  the  same  occurs. 
Blackman''*  has  shown  that  the  term  optimum  temperature  is  in- 
definite, since  at  certain  tempera tiires  physiological  processes  are  very 
rapid  for  a  time  but  then  slow  down,  due  to  the  introduction  of  a  time 
factor.  Tlie  maximum  rate  temperature  is  that  temperature  above 
which  a  time  factor  is  introduced  resulting  in  an  \iltiiiiate  retardation 
of  the  process. 

These  cardinal  temperatures  differ  somewhat  for  different  processes 
but  still  more  markedly  do  tliey  differ  for  the  same  process  in  different 


57  Ibid.,  Yearbook  15,  1916. 

ssOptim.i  and  Limiting  Factors,  Ann.  Bot.,  vol.  19  (1905),  pp.  .281-95. 


1919]       Coit-Uodgson:  Abnormal  Shedding  of  TTashinfjton  Navel  Orange  325 

organisms.  Thus  Howard''"  has  shown  witli  wheat  that  at  the  genni- 
nating  period  a  fall  of  10°  to  12"^  P  from  84°  to  72°  may  mean  the 
difference  between  success  and  failure  in  obtaining  a  stand,  since  the 
growth  rate  is  almost  inhibited  at  the  former  temperature.  On  the 
other  hand.  Cannon""  lias  shown  that  the  maxiinuni  rate  temperature 
for  the  mesquite,  Prosopis  velufina,  and  Opuntia  is  about  93°  P. 
Tobacco  is  another  plant  which  thrives  in  hot  soils.  Leitch"'  has  shown 
that  for  the  garden  pea,  Pisum  sativum,  85°  P  is  the  maximum  rate 
temperature  and  above  110°  F  no  gi-owth  whatever  occurs.  Appar- 
ently, as  in  tiie  ea.se  of  the  aeration  factor,  no  general  rule  for  these 
cardinal  temperatures  can  be  laid  down.  They  must  be  determined 
for  each  plant  separately.  Since  growtli  conditions  absorption  we 
are  justified  in  assuming  that  the  cardinal  temperatures  for  growth 
are  approximately  those  for  absorption. 

The  genus  Citrus,  as  mentioned  elsewhere,  is  native  to  the  trojjies, 
where  it  grew  in  the  shade  of  other  trees.  Under  the.se  conditions  the 
soil  was  damp  and  soil  temperatures  certainly  not  high.  It  therefore 
seems  logical  to  as.sume  that  the  temperatures  favorable  for  root  growth 
in  Citrus  are  not  very  high.  As  grown  under  clean  cultivation  in  the 
arid  .southwest  we  believe  that  tlie  absorbing  roots  are  subjected 
during  a  certain  portion  of  the  day  to  temperatures  above  the  optinnim 
and  that  during  such  period.s  absorption  is  actually  reduced. 

TABLE  11 
Son.  Temper.\tures  (F.)  at  Edison,  June  7,  1916 

A.M.  P.M. 

Uour  9:1.5  10:15  11:15  12:15  2:15  3:15  4:15  5:15 

Six-inch  dust  imilcli 80.6  84.2  88.2  92.3  94.1  96.0  99..5  99.0 

Fir-st  6  inches 77.0  78.3  80.0  84.2  89.6  88.8  88.6  87.0 

Second  6  inches 77.0  76.1  76.1  78.0  82.4  82.4  82.4  80.6 

Third  6  inches  76.1  7.").0  75.0  75.3  79.2  77.2  78.3  78.0 

Fourth  6  inches 74.3  74.3  74.6  74.6  77.2  76.6  77.0  77.0 

Six-inch  dust  mulch  in 

shade  of  tree 71.6  73.6  74.3  81.0  83.7  82..')  82.2  82.2 

To  obtain  an  idea  of  the  .soil  temperatures  prevailing  in  the 
upper  two  feet  of  soil  in  1916,  a  comparatively  cool  season,  we  made 
a  series  of  hourly  readings  at  six-inch  intervals.  These  may  be  found 
summarized  in  table  11.     This  table  shows  tiiat  during  the  afternoon 


ii"  Influence  of  Weather  on  YieM  of  Wheat,  Agr.  Jour.  India,  vol.  2  (1916), 
part  4. 

""  IJplation  of  the  Rate  of  Root  (Jrowth  in  Soedlings  of  Pronopis  velutina  to 
the  Temperature  of  the  Soil,  I'lant  World,  vol.  20  (1917),  ]ip.  320-33. 

"i  Some  Experiments  on  the  Influence  of  Temperature  on  the  Rate  of  Growth 
in  Pi.sjim  saliiitm,  Ann.  Bot.,  vol.  30  (1916),  pp.  25-46. 


326  University  of  California  Publications  in  Agricultural  Sciences      [Vol.  3 

the  temperature  of  this  upper  layer  of  soil  does  not  fall  below  75°  F. 
As  was  brought  out  previous]}',  under  clean  cultivation  practices  the 
absorbing  roots  of  citrus  trees  are  largely  located  in  the  upper  two 
feet  of  soil  (pi.  42).  It  therefore  seems  quite  probable  that  during 
the  afternoon  at  the  very  period  when  water  loss  by  transpiration  is 
greatest,  absorption  is  inhibited  by  high  .soil  temperatures.  A  study 
of  the  cardinal  temperatures  for  absorption  by  citras  roots,  which  is 
expected  to  throw  considerable  light  on  this  question,  is  now  under 
way  and  will  be  reported  on  later. 

But  granted  that  a  condition  of  physiological  drought  existed,  due 
to  the  action  of  the  factors  just  discussed,  still  the  citrus  tree  might 


Fig.  7.     Citrus   stoma   showing  maximum   opening.     From   orange  leaf  just 
reaching  full  size. 

maintain  itself  in  a  proper  water  balance  were  it  not  for  the  fact  that 
it  is  not  provided  with  efficient  means  of  conserving  its  water  by 
regulating  its  loss  through  transpiration.  A  preliminary  study  of  the 
relation  of  cuticular  tran.spiration  to  stomatal  water  loss  has  brought 
out  the  fact  that  from  40  to  50  per  cent  of  the  water  loss  from 
citrus  leaves  occurs  through  the  upper  epidermis  which  does  not  con- 
tain stomata.  These  studies  have  shown  that  the  young  leaves  are 
more  efficient  than  the  older  leaves  but  that  oven  the  youngest  leaves 
lose  as  much  as  25  per  cent  of  their  water  through  the  iipper  epidermis. 
A  study  of  the  stomatal  condition  in  citrus  leaves  has  brought  out 
some  interesting  facts.  By  the  use  of  Lloyd's  method"-  the  amplitude 
of  stomatal  movement  was  studied.  It  was  found  that  very  early  in 
the  life  of  the  leaf  the  stomata  lose  their  power  of  opening  and  closing 
and  remain  practically  closed  thereafter  (fig.  7).     In  some  cases  the 


0=  Physiology  of  Stomata,  Carnegie  Inst.  AVashingtou,  Puhl.  82  (1908),  p.  2G. 


1919]       Coit-Hodgson:  Abnormal  Shedding  of  Washington  Navel  Orange  327 

closure  is  not  complete  and  the  stomata  remain  slightly  open.  Heil- 
bronn"^  has  establislicd  this  same  condition  in  the  leaves  of  the 
Camclia.  It  is  interesting  to  note  in  this  regard  the  results  obtained 
by  Shreve'*  in  a  studj'  of  the  transpiration  of  rain-forest  plants  carried 
on  in  Jamaica. 

The  true  stomatal  transpiration  is  thus  found  to  be  from  42  to  48  per  cent 
of  the  total  water-loss  of  the  leaf.  The  close  relation  of  transpirational  behavior 
to  evaporation  is  thus  shown  to  have  its  basis  in  the  fact  that  rather  more  than 
half  of  the  water-loss  of  the  plant  goes  on  through  the  epidermal  surfaces.  .  .  . 
The  amplitude  of  stomatal  movement  in  rain-forest  plants  under  shade  con- 
ditions has  been  found  to  be  relatively  small.  .  .  .  The  weakness  of  the  move- 


Fig.  8.  Cross-section  of  stoma  from  old  coriaceous  orange  leaf.  Note  resin- 
ous deposit  in  the  substomatal  cavity. 

ments,  together  with  the  high  cuticular  water-loss,  serves  to  give  the  stomata  a 
very  negligible  role  as  regulators  of  transpiration  rate,  particularly  during  the 
daylight  hours. 

It  was  found  that  a  varying  percentage  of  citrus  stomata  are 
occluded  by  depo.sits  of  a  resinous,  gummy  nature  (fig.  8)  in  the  sub- 
stomatal cavity.  ITaberlandt"'  points  out  that  pliysiological  degenera- 
tiiin  of  stomata  takes  place  in  a  nuiiiber  of  shade-loving  hygrophytes, 
doubtless  because  members  of  these  ecological  classes  never  require 
much  protection  against  excessive  transpiration.  Therefore  it  can  be 
readily  appreciated  that  the  citrus  plant  has  relatively  little  control 


osBcr.  d.  dcut.  bot.  Ges.,  vol.  34  (1916),  pp.  22-31.  (Cited  from  Exp.  Sta. 
Record.) 

•x  The  Transpiration  Behavior  of  Rain-forest  Plants,  Ann.  Rep.  Dcpt.  Bot. 
Res.,  Carnegie  Inst.  Washington,  YearbooU  12  (1913),  [ip.  74-76. 

05  Physiological  Plant  Anatomy  (London  MacMillan,  1914),  p.  272. 


328 


University  of  California  Publications  in  Agricultural  Sciences      [Vol.  3 


over  its  water  loss.     This  coudition  itself  eou.stitutes  strong  evidcnoe 
of  its  tropical  origin. 

If  there  be  any  regulatory  action  upon  transpiration  it  should  be 
brought  out  in  a  study  of  the  transpiration  curve  as  compared  to  the 
evaporation  curve.  These  two  curves  for  a  typical  day  in  July  are 
shown  in  figure  9,  and  it  will  be  seen  that  the  general  form  is  very 
similar  and  that  the  maxima  of  the  two  were  reached  at  the  same 


4  p.  M.      JULY 


12  p.  .M 


JULY 


12  .M. 


Fig.  9.  Comparison  of  Citrus  transpiration  curves  witli  the  evaporation 
curve  for  the  same  period.  Nos.  1,  2,  3,  and  4  are  transpiration  curves  obtained 
bv  the  potometer  method.  No.  5  is  the  evaporation  curve  obtained  from  a 
Livingston  white  cyliBdrical  porous  cup  atmometer.  Ordinates  represent  water 
loss  in  cc;  abscissae,  hours  of  the  day. 

period.     "Were  there  any  regulatory  action   the  transpiration   curve 
should  reach  its  maximum  some  time  before  the  evaporation  curve. 


Susceptibility  of  Citrus  Varieties  to  Abscission 

It  is  well  Imown  that  when  grown  under  similar  conditions  the 
Valencia  variety  of  orange  and  the  pomelo  do  not  shed  the  young 
fruits  in  anything  like  the  same  proportion  as  the  Washington  Navel. 


1919]       Cuit-Hodffson :  Abnormal  Shedding  of  Washington  Navd  Orange  329 

If  the  stimulus  leading  to  abscission  be  abnormal  water  relations,  why 
then  do  not  these  two  other  members  of  the  genus  shed  their  fruits 
to  the  same  extent  as  the  navel  variety?  Our  observations  made  in  the 
field  in  orchards  where  those  varieties  are  mixed  have  shown  that  such 
is  not  the  case,  and  experiments  performed  in  our  laboratories  have 
shown  that  abscission  is  much  more  easily  induced  in  the  navel  variety 
than  in  the  others.  Shoots  bearing  flowers  and  young  fruits  of  each 
variety  have  been  ])laeed  in  moist  chambers  and  kept  at  room  tciiqiera- 
ture.  In  the  case  of  the  navel  variety  abscission  of  all  the  flowers  and 
fruits  has  invariably  occurred  within  sixty  hours,  while  in  the 
Valencia  variety  and  with  lemons  frequently  no  absci.s.sion  occurred 
within  five  to  eight  days.  Apparently  the  navel  variety  is  nuich  more 
susceptible  to  stimuli  which  lead  to  absci.ssiou.  In  this  coiuiection  it 
seems  desirable  to  call  attention  to  the  fact  that  other  investigators 
have  found  in  the  case  of  hybrids  ab.scission  is  much  more  prevalent 
and  nuich  more  easily  brought  about  than  in  the  ease  of  the  parent 
varieties.  Thus  Goodspeed  and  Kendall"''  have  shown  that  in  the  ea.se 
of  certain  tobacco  crosses  in  which  (mly  a  small  proportion  of  the 
ovules  are  normally  matured  and  capable  of  fertilization,  which  con- 
dition obtains  in  the  navel  orange  variety,  practically  all  the  fiowers 
and  young  fruits  are  abscissed.  May  not  this  sensitiveness  to  stimuli 
which  cause  abscission  constitute  further  evidence  that  the  Washing- 
ton Xavel  variety  is  of  hybrid  origin? 

Methods  op  Amelioration 

From  the  preceding  discussion  it  is  obvious  that  all  methods  of 
preventing  the  June  drop  of  our  present  strains  of  Washington  Navel 
oranges  nuist  be  in  the  nature  of  modifying  the  environmental  complex 
either  above  ground,  below  frround.  or.  as  is  usually  the  case.  both. 

If  the  cause  underlying  these  water  deficits  lies  in  the  asperity  of 
the  atmospheric  complex  then  practices  tending  to  ameliorate  climatic 
conditions  should  work  out  to  produce  heavier  crops.  Such  has  been 
found  to  be  the  ease.  The  |>hniting  of  windbreaks  to  prevent  th(> 
di.ssipation  of  blankets  of  moist  air;  a  moderate  winter  i)runiug  to 
reduce  the  total  leaf  surface  area ;  aiul  the  planting  of  intercroi)s.  such 
as  alfalfa,  sweet  clover,  or  buckwheat,  which  transpire  large  amounts 
of  water  viipor;  all  these  are  methods  of  modifying  the  atmospheric 
environiueiital  complex. 


"»  On  the  Partial  Storility  of  Nirntiana  Hvtiriils  niaile  with  A'.  siiU'rutris  as  a 
Parent,  III:  An  Accomit  of  tlip  Mode  of  Floral  Abscission  in  the  F,  Species 
Hybrids,  Univ.  Calif.  I'ubl.  Bot.,  vol.  5  (1916),  i>p.  29.V90. 


330  University  of  California  Publications  in  Agricultural  Sciences      [Vol.  3 

In  this  connection  it  should  be  emphasized  that  the  beneficial  effect 
of  a  summer  cover  crop  does  not  seem  to  be  due  so  much  to  the  raising 
of  the  average  humiditj-  as  it  does  to  the  buffer  effect  which  it  plays 
when  sudden  extremes  in  climatic  conditions  are  experienced.  The 
increase  in  the  average  humidity  occasioned  by  the  use  of  a  summer 
cover  crop  is  probably  considerably  smaller  than  the  difference  which 
may  exist  from  one  season  to  the  next.  It  does  not  seem  so  important 
that  the  average  humidity  has  been  increased  somewhat  by  its  use  as 
that  when  sudden  hot,  dry  spells  are  experienced  their  effect  is 
modified  by  the  use  of  such  a  crop.  This  would  seem  also  to  explain 
the  effect  of  the  straw  mulch  which  of  course  does  not  affect  the  atmo- 
spheric humidity  to  any  extent. 

If  the  limiting  factor  causing  these  abnormal  water  relations  be 
high  soil  temperatures  then  methods  of  orchard  management  which 
will  reduce  such  temperatures  may  be  expected  to  result  in  heavier 
crops.  Such  practices  as  mulching  and  the  growing  of  intercrops  are 
known  to  reduce  the  soil  temperatures.  ^Moreover,  such  practices  in 
many  cases  have  resulted  in  notably  heavier  yields.  The  junior  author 
had  under  observation  a  twenty-acre  orchard  in  the  Oroville  district 
in  the  1917  season.  This  tract  was  planted  out  to  purple  vetch  in  the 
late  fall  and  was  not  plowed  until  the  following  June.  It  was  heavily 
irrigated  during  April  and  ]May.  Although  situated  in  a  most  exposed 
position  this  orchard  bore  a  much  better  crop  than  any  other  orchard 
in  this  district,  notwithstanding  the  extremely  heavy  fall  of  fruits 
experienced  in  this  season.  It  is  po.ssible  that  the  heavj'  crops  borne 
at  the  Kellogg  place  are  partlj^  attributable  to  a  reduction  in  soil 
temperature  during  the  growing  season. 

Some  datxi  have  been  published  on  the  effect  of  straw  mulches  on 
the  setting  of  Navel  oranges.  Briggs,  Jensen,  and  McLane"'  report 
as  follows: 

The  set  of  fruit  was  very  light  throughout  the  Riverside  district  in  1915, 
owing  apparently  to  cold  weather  following  the  bloom.  In  the  Sunny  Mountain 
tract,  where  the  mulched  basins  were  first  installed  in  1913,  the  average  number 
of  oranges  per  tree  on  the  check  trees  in  191.5  was  116,  while  on  the  mulched- 
basin  trees  the  average  number  of  oranges  per  tree  was  281,  or  two  and  one  half 
times  as  many  as  on  the  check  trees. 

Similar  results  are  reported  from  other  tracts.  It  .should  be  remem- 
bered, however,  that  the  trees  used  in  this  work  were  not  healthy  but 
were  badly  mottled,  and  the  increased  setting  mav  be  attributable  to 


8'  The  Mulched-Basin  System  of  Irrigated  Citrus  Culture,  V.  S.  Dept.  Agr., 
Bull.  499  (1917),  p.  30. 


1919]      Coit-Hodgson :  Abnormal  Shedding  of  Washington  Navel  Orange  331 

their  improved  health  broiight  about  by  better  soil  moisture  and  humus 
conditions  as  well  as  improved  temperature  conditions.  It  lias  not  yet 
been  satisfactorily  shown  that  the  mulchcd-basin  system  alone  will 
reduce  the  amount  of  drop  on  healthy  trees,  although  in  the  light  of 
the  discussion  above  we  believe  it  probable. 

The  determination  of  the  specific  factor,  if  it  be  a  single  factor, 
which  produces  the  abnormal  water  relations  established,  is  yet  to  be 
made.  It  is  hoped  that  investigations  planned  for  the  coming  season 
may  aid  in  solving  this  question.  The  orchard  management  practices 
described  above  which  result  in  heavier  crops,  unfortunately  for  in- 
vestigational purposes,  involve  the  modification  of  both  the  above- 
ground  and  under-ground  environmental  complex. 

The  fact  that  by  proper  means  man  is  able  to  change  the  climatic 
conditions  from  those  obtaining  at  Tucson,  Arizona,  to  those  at  iliami, 
Florida,  within  the  space  of  a  half  mile,  augurs  well  for  the  successful 
control  of  the  June  drop.  ]\Ieasures  of  an  anticipatory  nature  lie  in 
the  proper  selection  of  the  site  before  planting.  The  exposure  to  pre- 
vailing winds,  the  nearness  to  large  irrigated  tracts,  the  possibility  of 
planting  windbreaks;  all  these  should  be  considered  in  the  selection 
of  a  site  for  a  Navel  orange  grove.  Growers  should  accustom  them- 
selves to  thinking  of  climate  not  in  terms  of  great  valleys  and  states 
but  in  .strictly  local  terms.  As  has  been  pointed  out  above,  the  judicious 
selection  of  tiie  site,  coupled  with  proper  metliods  of  orchard  practice, 
make  it  passible  to  secure  marked  modiiications  in  our  arid  climate. 
The  i|Ui'stiiiii  (if  the  advisal)i]ity  of  the  nu'asures  suggested  is  purely 
one  of  farm  economics  and  does  not  lie  within  the  province  of  this 
paper. 

In  view  of  the  relatively  small  amount  of  shedding  which  is  con- 
nected with  the  Allcrimria  fungus  alone  and  because  of  the  peculiar 
manner  of  infection  the  authors  are  led  to  believe  that  spraying  with 
fungicides  for  the  June  drop  will  hardly  pay  for  the  materials  and 
labor  involved. 

Another  promising  line  of  investigation  looking  toward  control  of 
the  June  drop  lies  in  the  selection  and  propagation  of  dry  heat  resist- 
ant strains  of  the  Washington  Navel  variety.  This  variety,  it  is  well 
knowTi,  is  constantly  throwing  off  bud  sports  or  mutations  and  i1  is 
entirely  possible  that  mutations  maj'  arise  which  are  le.ss  sensitive  to 
abscission  stinuili,  but  at  the  same  time  satisfactory  otherwi.se.  Every 
grower  should  be  on  the  lookout  for  such  strains. 


332  Vniversity  of  California  Puhlications  in  Agricultural  Sciences      [Vol.  3 

SUMMARY 

1.  Citrus  trees  as  grown  in  the  interior  valleys  of  the  arid  south- 
west are  subject  to  an  environment  entirely  abnormal  to  them  in  their 
natural  habitat. 

2.  Moreover,  the  principal  variety  grown  in  these  regions,  the 
Washington  Navel  orange,  is  itself  decidedly  erratic  and  unstable. 

3.  Among  other  troubles  incident  to  the  abnormal  climatic  con- 
ditions is  that  heavy  dropping  of  tlie  young  fruits,  with  consequent 
light  crops,  known  popularly  as  the  June  drop. 

4.  A  study  of  the  shedding  has  established  the  fact  that  it  con- 
stitutes true  ab.sci.ssion,  involving  the  separation  of  living  cells  along 
the  plane  of  the  middle  lamellae. 

5.  Exhaustive  investigations  as  to  the  stimulus  or  stimuli  responsible 
for  the  abscission  have  narrowed  them  down  to  two :  a  fungus.  Altcr- 
naria  citri  E.  and  P.,  and  climatic  conditions. 

6.  It  is  considered  highly  probable  that  a  certain  varying  per  cent 
of  the  drop,  occurring  relatively  late  in  the  season,  is  brought  about 
by  the  stimulation  of  this  fungus,  which  is  also  responsible  for  a  black 
rot  of  those  infected  fruits  which  remain  on  the  trees  to  maturity. 

7.  This  fungus  is  of  very  wide  distribution  and  infection  of  the 
young  fruits  is  made  possible  thrmigh  the  peculiar  structure  of  the 
navel  orange. 

8.  The  amount  of  infection  is  dependent  upon  weather  conditions 
and  the  more  or  less  fortuitous  configuration  of  the  navel  end  of  the 
young  fruits. 

9.  On  account  of  the  peculiar  manner  of  infection  and  the  rela- 
tively .small  amount  of  shedding  diu;  to  the  fungus,  spi'aying  will 
probably  not  pay  for  the  labor  and  materials  involved. 

10.  By  far  the  greater  part  of  the  shedding,  which  occurs  earlier 
in  the  season,  is  due  to  a  stimulus  to  absci.ssion  arising  from  daily 
water  deficits  in  the  young  developing  fruits,  resulting  from  the 
asperity  of  the  climatic,  complex  to  which  the  trees  are  subject. 

11.  The  principal  factor  in  causing  these  abnormal  water  deficits 
lies  in  the  fact  that  citrus  trees  are  not  adapted  to  with.stand.ing  the 
heavy  water  loss  incident  to  the  desert  conditions  under  which  they 
are  grown.  The  amplitude  of  .stomatal  movement  is  small  and  cutic- 
ular  transpiration  very  high. 

12.  It  is  further  believed  that  under  the  prevalent  clean  cultivation 
practice,  the  soil  temperatures  during  a  part  of  the  day  are  so  high  as 


1919]      Coit-Hodgson :  Abnormal  Shedding  of  Washington  Navel  Orange  333 

to  result  ill  the  inhibition  of  al)sorption  at  the  very  time  of  day  that 
water  h).ss  l)y  transpiration  is  greatest. 

13.  It  has  been  found  possible  to  modify  climatic  conditions  in  an 
orchard  so  as  to  set  crops  in  every  way  comparable  with  those  produced 
in  much  more  climatically  favored  citrus  districts. 

14.  Under  these  modified  climatic  conditions  the  abnormal  water 
relations  referred  to  apparently  do  not  occur. 

15.  Practical  means  of  amelioration  lie  in  heavier  and  more  fre- 
'  quent  irrigation,  the  planting  of  intercrops,  mulching  with  straw  and 

other  materials,  protection  by  means  of  windbreaks,  and  a  reduction  of 
leaf  area  by  moderate  winter  pruning. 

16.  Pleasures  of  an  anticipatory  nature  lie  in  the  judicious  selec- 
tion of  the  site  for  the  orchard  with  reference  to  its  exposure,  nearness 
to  large  irrigated  bodies  of  land,  and  other  features  calculated  to 
ameliorate  climatic  conditions. 

17.  Orchardists  should  be  on  the  lookout  for  mutant  strains  which 
are  dry  heat  resistant  and  satisfactory  in  other  features. 

Tliis  investigation  had  its  inception  with  the  senior  autlior,  who 
began  the  experimental  work  in  March.  1916.  In  May,  1916,  the  junior 
author  became  connected  with  the  Division  of  Citriculture  and  has  been 
associated  in  the  study  of  this  problem  ever  since.  Early  in  the  in- 
vestigation it  became  evident  that  there  were  at  least  two  distinct 
promising  lines  of  in(|uiry  involved  in  the  problem.  The  first,  having 
to  do  with  the  relation  of  a  certain  almo.st  ever-present  fungus  to  the 
falling  of  the  young  fruits,  is  largely  the  work  of  the  senior  author. 
The  .second,  having  to  do  with  the  relation  of  the  shedding  to  environ- 
mental conditions,  although  originating  with  the  senior  author  and 
receiving  constant  study  by  him,  constituted  the  main  problem  of  the 
junior  author,  who  moreover  is  resptmsible  for  the  histological  work 
involved  in  the  investigation.  The  combination  of  attack,  both  on  the 
pathological  and  physiological  side,  has  given  most  satisfactory  results 
and  it  is  the  belief  of  the  authors  that  when  investigated  in  a  some- 
what similar  manner  many  of  tuir  so-called  "physiological  diseases" 
may  be  better  understood. 

The  authors  wish  to  ackowledge  their  indebtedness  to  Drs.  F.  E. 
Lloyd,  W.  A.  Canndii.  T.  II.  Gondspeed.  and  ('.  B.  Li])iiian  for  sugges- 
tions and  assistance,  and  to  ^Ir.  \V,  AV.  Worden  and  Dr.  C.  W.  Kellogg 
for  kindly  cooperation  in  placing  tlnii-  orchard  facilities  at  their 
disposal. 

Transmitted  Jamtary  17,  1918. 


EXPLANATION  OF  PLATES 

PLATE  25 

The  Navel  orange  orchard  of  the  Edison  Land  and  Water  Company,  where 
much  of  the  experimental  work  was  done. 


[334] 


I 


i 


UNIV.    CALIF.    PUBi..    AGR.    SCI.   VOL.    3 


[  COIT-HODGSON  ]    PLATE    25 


PLATE  2(i 

Part  of  the  Kellogg  orchard  at  East  BaUersfield,  showing  heavy  stand  of 
alfalfa  (just  cut)  between  trees  and  also  heavy  crop  of  fruit.  Photographed 
November  25,  1917. 


[336] 


UNIV.    CALIF.    PUBL.   AGR.    SCI.    VOL.    3 


I  COIT-HODGSON  ]    PLATE    26 


PLATE  27 
Typical  Washington  Navel  tree  in  San  Joaquin  Valley,  showing  heavy  bloom. 


[338] 


UNIV.    CALIF.    PUBL.   AGR.    SCI.    VOL.    3 


I  COIT-HODGSON  ]    PLATE   27 


W'^ 

T^59 

^ 

1% 

PLATK  2S 
Nearer  view  of  same  tree,  showing  details  of  heavy  bloom. 


[340] 


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PLATE  29 
One  branch  with  leaves  removed,  showing  large  number  of  buds  produced. 


[342] 


UNIV.    CALIF.    PUBL.   AGR.    SCI.    VOL.    3 


[  COIT-HODGSON  1    PLATE   29 


PLATE  80 

Typical  abscissed  fruits.  Those  to  the  rifrht  abscissed  at  the  base  of  the 
pedicel,  those  to  the  left  at  the  base  of  the  ovary.  The  two  in  the  center  are 
healthy  fruits  picked  from  the  tree  for  comparison. 


[344] 


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PLATE  31 

Small  dead  orange  persisting  though  abscissed  both  at  base  of  ovary  and 
pedicel.  Large  fruit  safely  through  both  abscission  periods.  The  dead  style 
abscissed  much  earlier  but  was  retained  in  position  by  the  ragged  nature  of  the 
break. 


[346] 


UNIV.    CALIF.    PU8L.    AGR.    SCI.    VOL.    3 


COIT-HODGSON  1    PLATE   31 


PLATE  32 
The  serious  wounds  produced  b}'  katydids  which  never  result  in  abscission. 


[348] 


UNIV.    CALIF.    PUBL.    AGR.    SCI.   VOL.    3  [  COIT-HODGSON  ]    PLATE   32 


V 


^  y  y  * 


10 


PLATE  33 
Terminal  and  axillary  fruits. 


[350] 


UNIV.    CALIF.    PUBL.   AGR.    SCI.   VOL.    3  [  COIT-HODGSON  )    PLATE   33 


PLATE  34 

Apical  end  of  ovary  of  Navel  orange  just  after  the  style  has  been  shed. 
Enlarged  10  diameters.     Notice  the  ragged  condition  of  the  stylar  scar. 


[352] 


UNIV.    CALIF.    PUBL.    AGR.    SCI.   VOL.    3 


[COIT-HODGSON  1    PLATE    34 


PLATE  35 

Large  late   drops   showing  discolored   area   beneath   the   navel,   caused  by 
infection  with  Alternaria  citri. 


[354] 


UNIV.    CALIF.    PUBL.   AGR.    SCI.   VOL.    3  [  COIT-HODGSON  )    PLATE   35 


PLATE  36 
Photomicrograph  of  Alternaria  citri,  showing  the  spores  borne  in  long  chains. 


[356] 


UNIV.   CALIF.    PUBL.    AGR.    SCI.   VOL.    3  (  COIT-HODGSON  ]    PLATE   36 


/^    ' 


PLATE  37 

Young  Navel  oranges,  showing  the  ragged  break  of  the  style.     Enlarged 
2  diameters. 


[358] 


UNIV.    CALIF.    PUBL.    AGR.    SCI.   VOL.    3  [  COIT-HODGSON  1    PLATE   37 


--» 


©  1  ^ 


m 

^ 


-^^ 


PLATE  38 
Mummified  oranges  infected  with  AHernaria  citri.     Gathered  under  tree. 


[360] 


UNIV.   CALIF.    PUBL.    AGR.    SCI.    ^/OL.    3  I  COIT-HODGSON  1    PLATE    38 


PLATK  :i<l 


Small  Valencia  orange,  showing  clean  break  between  the  base  of  the  style 
and  the  ovary.    Jinlarged  10  diameters.     Compare  with  plate  34. 


[362] 


UNIV. 


CALIF.    PUBL.    AGR.    SCI.   VOL.    3 


[COIT-HODGSON  1    PLATE   39 


PLATE  40 

Showing  the  method  of  enclosing  orange  trees  under  the  tents  of  cheesecloth 
in  order  that  bees  may  be  included  in  one  and  excluded  from  the  other.  The 
tree  in  foreground  shows  the  method  of  covering  inoculated  flowers  with  paper 
sacks. 


[364] 


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PLATE  41 
The  Livingston  wliite  porous  cup  atmometer  as  set  up  at  our  Desert  station. 


[366] 


UNIV.    CALIF.    PUBL.   AGR.    SCI.   VOL.    3 


[COIT-HODGSON  1    PLATE   41 


PLATE  42 

Distribution   of  orange  roots  by  six-incla   lajers   at  Edison   station.     Clean 
cultivation. 


[368] 


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tJKIVBEBITT  OP  OALIFOENIA  PUBLICATIONS— (Continued) 

6.  Oontrlbutiona  to  tba  Knowledge  of  the  Califoroia  Species  of  Orusta- 

ceous   Corallines,    I,    by    Maurica    Barstow   Nichols.    Pp.    341-348; 

plate  9.    December,  1908  _ _ .10 

0.  Contributions  to  the  Knowledge  of  the  California  Species  of  OmstA- 
ccous  CoralllneB.  n,  by  Maurice  Barstow  Nichols.  Pp.  349-370; 
plates  10-13.    April,  1909  __      M 

7.  New  Chlorophyceae  from  California,  by  Nathaniel  Lyon  Gardner.    Pp. 

371-375;  plate  14.    April,  1909  _      40 

8.  Plantae  Mexlcanae  Purpuslanae,  by  T.  S.  Brandegee.      Pp.  377-306. 

May,    1909    _ _ _ „      .IB 

Index,  pp.  397-400. 

VoL  4.    1.  Studies  In  Ornamental  Trees  and  Shrubs,  by  Harvey  Monroe  Hall.    Pp. 

1-7-1;  plates  1-11;  15  test-figures.    March,  1910  _ .71 

5.  Gracllariophlla,  a  New  Parasite  on  prarilaria  confervoides,  by  Harriet 

L.  WUson.    Pp.  75-84;  plates  12-13.    May,  1910  10 

8,  Plantae  Mexlcanae  Purpuslanae,  n,  by  T.  S.  Biandegee.     Pp.  86-95. 

May,    1910    „ - - 10 

4.  Leuvenia,  a  New  Genus  of  Flagellates,  by  N.  L.  Gardner.    Pp.  97-106; 

plate  14.    May,  1910  _ „ _ M 

B.  The  Genus  Sphaerosnma,  by  William   Albert  Setchell.     Pp.   107-120; 

plate  15.    May,  1910  „ „      .10 

6.  Variations  in  Nuclear  Extrusion  Among  the  Fucaceae,  by  Nathaniel 

Lyon  Gardner.    Pp.  121-136;  plates  1617.    August,  1910  _ 18 

7.  The  Nature  of  the  Carpostomes  In  the  Cystocarp  of  Ahnfeldtia  gigarti- 

noidei,  by  Ada  Sara  McFadden.    Pp.  137-142;  plate  18.    FebmarT', 

1911    - „ - - -       .08 

8.  On  a  CoUwndasya  from  Southern  California,  by  Mabel  Efile  McFadden. 

Pp.  143-150;  plate  19.    February,  1911  .„ „ .08 

B.  Fnictlfidatlon  of  Mncroryttis,  by  Edna  Juanlta  Ho&maa.    Pp.  161-168; 

plate  20.     February,  1911  — _      .08 

10.  ErythrophyUum  delesserioides  J.  Ag.,  by  Wilfred  Charles  Twlss.     Pp. 

159-176;  plates  21-24.    March,  1911  _ 18 

11.  Plantae  Mexlcanae  Purpuslanae,  III,  by  T.  S.  Brandegee.    Pp.  177-194. 

July,    1911    — _- -      .18 

IS.  New  and  Noteworthy  California  Plants,  I,  by  Harvey  Monroe  Hall. 

Pp.  195-208.     March,  1912  _ M 

15.  Die  Hydrophyllaceen  der  Sierra  Nevada,  by  August  Brand.    Pp.  209- 

227.     March,   1912 ^..      M 

14.  Algae  Novae  et  Minns  Cognltae,  I,  by  William  Albert  Setchell.     Pp. 

229-268;  plates  25-31.     May,  1912  .._ .40 

16.  Plantae  Mexlcanae  Purpuslanae,  IV,  by  Townshend  Stlth  Brandegee. 

Pp.  269-281.     June  28,  1912  _ —       .18 

16.  Comparative    Development    of    the    Cystocarps    of    Antitlunnnion    and 

I'rionitis,  by  Lyman  L.  Dalnes.    Pp.  283-302;  plates  32-34.    March, 

1912    - -       M 

17.  Fungus  Galls  of  Cystoieira  and  Halidrys,  by  Lulu  M.  Estee.  Pp.  306-316; 

plate  35.     March,  1913  _       J.0 

18.  New  Fucaceae,  by  Nathaniel  L.  Gardner.     Pp.  317-374;  plates  S6-6S. 

April,   1913  _ .78 

10.  Plantae  Mexlcanae  Purpuslanae,  V,  by  Townshend  Stlth  Brandegee. 

Pp.  375-388.     June,   1913  .18 

Index,  pp.  389-397. 

Vol  8.    1.  Studies  In  Nicotiana  I,  by  William  A.  SetcheU.    Pp.  1-86;  plates  1-28. 

December,  1912  _ US 

2.  Quantitative  Studies  of  Inheritance  In  Nicotiana  Hybrids,  by  Thomas 

H.  Goodspeed.    Pp.  87-188;  plates  29-34.    December,  1912  _     1.00 

8.  Quantitative  Studies  of  Inheritance  in  Nicotiana  Hybrids  II,  by  Thomaa 

H.  Goodgpeed.     Pp.  169-188.     January,  1913  .80 

4.  On  the  Partial  Sterility  of  Nicotiana  Hybrids  made  with  N.  Sylveatrit 

as  a  Parent,  by  Thomas  E.  Goodspeed.    Pp.  189-198.    March,  1913.._       M 

6.  Notes  on  the  Germination  of  Tobacco  Seed,  by  Thomas  Harper  Good- 

speed.    Pp.  199-222.    May,  1913  _ _.      .18 

0.  Quantitative   Studies   of   Inheritance   In  Nicotiana   Hybrids,    HI,   by 

Thomas  Harper  Goodspeed.    Pp   223-231.    April,  1915  _ 10 

7.  Notes  on  the  Geniilnatlon  of  Tobacco  Seed,  II,  by  Thomas  Harper  Good- 

speed.     Pp.  233-248.     June,  1915  IB 

8.  Parthenogenesis,    Parthenocarpy,    and   Phenospermy   tn   Nicotiana,   by 

Thomas  Harper  Goodspeed.    Pp.  249-272,  plate  35.    July,  1915 .28 


XnXTVBBMTn  OF  CAIJFOKKIA  PUBLIOATIOK&— (Continued) 

0.  On  the  Partial  Sterility  of  Nicotiana  Hybrids  made  with  N.  sylvestrit 
as  a  Parent,  n,  by  T.  H.  Ooodspeed  and  A.  H.  Ayres.  Pp.  273-292, 
pL  36.    October,  1916  „_ _ _ _ —       .30 

10.  On  the  Partial  Sterility  of  Nicotiana  Hybrids  made  with  N.  sylvestri* 

as  a  Parent,  III:  An  Account  of  the  Mode  of  Floral  Abscission  In  the 
F,  Species  Hybrids,  by  T.  H.  Ooodspeed  and  J.  N.  Kendall.  Pp.  293- 
299.    November,  1916 _ _ .08 

11.  The  Nature  of  the  F,  Species  Hybrids  between  Nicotwna  sylvestris  and 

Varieties  of  Nicotiana  Tabacum,  with  Special  Eeference  to  the  Con- 
ception of  Beaction  System  Contrasts  in  Heredity,  by  T.  H.  Oood- 
speed and  E.  E.  Clausen.    Pp.  301-346,  pis.  37-48.    January,  1917 45 

12.  Abscission  of  Flowers  and  Fnilts  in  Solanaceae  with  Special  Eeference 

to  Nicotiana,  by  John  N.  Kendall.  Pp.  347-428,  pis.  49-53.  March, 
1918 85 

13.  Controlled  PoUlnation  in  Nicotiana,  by  Thomas  H.  Ooodspeed  and  Pirie 

Davidson.    Pp.  429-434.    August,  1918 10 

14.  An  Apparatus  for  Flower  Measurement,  by  T.  H.  Ooodspeed  and  E.  E. 

Clausen.    Pp.  435-437,  plate  54,  1  text  figure.    September,'1918 05 

15.  Note  on  the  Effects  of  Uluniinating  Gas  and  its  Constituents  in  Causing 

Abscission  of  Flowers  in  Nicotiana  aud  Citrus,  by  T.  H.  Ooodspeed, 

J.  M.  McGee,  and  B.  Vf.  Hodgson.    Pp.  439-450.    December,  1918 15 

Vol.  6,    1.  Parasitic  Florideae,  I,  by  WilUam  Albert  Setchell.    Pp.  1-34,  plates  1-6. 

April,   1914  ...._ — - —       .88 

2.  Phytcmwrula  reguXaris,  a  Symmetrical  Protophyte  related  to  Coelastrum, 

by  Charles  Atwood  Kofoid      Pp.  35-40,  plate  7    April,  1914 „      .06 

3.  Variation  in  Oenothera  ovata,  by  Kathertne  Layne  Brandegee.    Pp.  41- 

60,  plates  8-9.     June,  1914  .- .10 

4.  Plantae  Mexlcanae  Purpusianae,  VI,  by  Townshend  Stith  Brandegee. 

Pp.  61-77.    August,  1914  .88 

5.  The  Scinaia  Assemblage,  by  William  A.  Setchell.    Pp.  79-152,  plates  10- 

16.     October,   1914  — - .76 

6.  Notes  on  Pacific  Coast  Algae,  I:  Pylaiella  Postelsiae,  n.  sp.,  a  New  Type 

in  the  Genus  Pylaiella,  by  Carl  Skottaberg.  Pp.  153-164,  plates  17-19. 
May,  1915  - - -      .10 

7.  New  and  Noteworthy  Califorilan  Plants,  n,  by  Harvey  Monroe  HaU. 

Pp.  165-176,  plate  20.    October,  1915 —       .10 

8.  Plantae  Mesicanae  Purpusianae,  VII,  by  T.  8.  Brandegee.    Pp.  177-197. 

October,  1915  20 

9.  Floral  Eolations  among  the  Galapagos  Islands,  by  A.  L.  Kroeber.    Pp. 

199-220.     March,   1916   .20 

10.  The    Comparative    Histology    of    Certain    Califomian    Boletaceae,    by 

Harry  S.  Yates.    Pp.  221-274,  plates  21-25.    February,  1916  _      .60 

11.  A  revision  of  the  Tuberales  of  California,  by  Helen  Margaret  Gllkey, 

Pp.  275-356,  plates  26-30.    March,  1916  80 

12.  Species  Novae  vel  Minus  Gognitae,  by  T.  S.  Brandegee.     Pp.  357-361. 

April,   1916  - 06 

13.  Plantae  Mexicanae  Pumusianae,  VIII,  by  Townshend  Stith  Brandegee. 

Pp.  363-375.    March,  1917  IB 

14.  New  Pacific  Coast  Marine  Algae,  I,  by  Nathaniel  Lyon  Gardner.    Pp. 

377-416,  plates  31-35.    June,  1917  40 

15.  An  Account  of  the  Mode  of  Foliar  Abscission  in  Citrus,  by  Eobert  W. 

Hodgson.    Pp.  417-428.    February,  1918  10 

16.  New  Pacific  Coast  Marine  Algae,  II,  by  Nathaniel  Lyon  Gardner.    Pp. 

429-454,  plates  36-37.    July,  1918  25 

17.  New  Pacific  Coast  Marine  Algae,  IH,  by  Nathaniel  Lyon  Gardner.    Pp. 

455-486,  plates  38-41.    December,  1918  35 

VoL  7.    Notes  on  the  Califomian  Species  of  Trillium. 

1.  A  Eeport  of  the  General  Eesulta  of  Field  and  Garden  Studies,  1911- 

1916,  by  Thomas  Harper  Ooodspeed  and  Eobert  Percy  Brandt.    Pp. 
1-24,  pis.  1-4.    October,  1916  _ - _      JJ6 

2.  The  Nature  and  Occurrence  of  Undeveloped  Flowers,  by  Thomas  Harper 

Ooodspeed  and  Eobert  Percy  Brandt.    Pp.  25-38,  pis.  5-6.    October, 
1916 „ - -       .16 

3.  Seasonal  Changes  in  Trillium  Species  with  Special  Eeference  to  the 

Eeproductive  Tissues,  by  Eobert  Percy  Brandt.    Pp.  39-68,  pis.  7-10. 

December,  1916 ~ -      .30 

i.  Teratological  Variations  of  Trillium  sessile  var.  giganteum,  by  Thomas 

Harper  Ooodspeed.    Pp.  69-100,  pis.  11-17.    January,  1917 .30 

hi. I 


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